RPC-over-RDMA Version One Implementation Experience
draft-cel-nfsv4-rfc5666-implementation-experience-01
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draft-cel-nfsv4-rfc5666-implementation-experience-01
NFSv4 C. Lever
Internet-Draft Oracle
Intended status: Informational October 9, 2015
Expires: April 11, 2016
RPC-over-RDMA Version One Implementation Experience
draft-cel-nfsv4-rfc5666-implementation-experience-01
Abstract
This document details experiences and challenges implementing the
RPC-over-RDMA Version One protocol. Specification changes are
recommended to address avoidable interoperability failures.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 11, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Purpose Of This Document . . . . . . . . . . . . . . . . 4
1.3. Updating RFC 5666 . . . . . . . . . . . . . . . . . . . . 4
1.4. Scope Of This Document . . . . . . . . . . . . . . . . . 5
2. RPC-Over-RDMA Essentials . . . . . . . . . . . . . . . . . . 5
2.1. Arguments And Results . . . . . . . . . . . . . . . . . . 5
2.2. Remote Direct Memory Access . . . . . . . . . . . . . . . 6
2.2.1. Small Data Transfers . . . . . . . . . . . . . . . . 6
2.2.2. Large Data Transfers . . . . . . . . . . . . . . . . 7
2.3. Transfer Models . . . . . . . . . . . . . . . . . . . . . 7
2.3.1. Read-Read . . . . . . . . . . . . . . . . . . . . . . 7
2.3.2. Write-Write . . . . . . . . . . . . . . . . . . . . . 7
2.3.3. Read-Write . . . . . . . . . . . . . . . . . . . . . 8
2.4. Upper Layer Binding Specifications . . . . . . . . . . . 8
2.5. On-The-Wire Protocol . . . . . . . . . . . . . . . . . . 8
2.5.1. Inline Operation . . . . . . . . . . . . . . . . . . 8
2.5.2. RDMA Segment . . . . . . . . . . . . . . . . . . . . 11
2.5.3. Read Chunk . . . . . . . . . . . . . . . . . . . . . 11
2.5.4. Write Chunk . . . . . . . . . . . . . . . . . . . . . 12
2.5.5. Read List . . . . . . . . . . . . . . . . . . . . . . 13
2.5.6. Write List . . . . . . . . . . . . . . . . . . . . . 13
2.5.7. Position Zero Read Chunk . . . . . . . . . . . . . . 14
2.5.8. Reply Chunk . . . . . . . . . . . . . . . . . . . . . 14
3. Specification Issues . . . . . . . . . . . . . . . . . . . . 14
3.1. XDR Clarifications . . . . . . . . . . . . . . . . . . . 14
3.1.1. Recommendations . . . . . . . . . . . . . . . . . . . 16
3.2. The Position Zero Read Chunk . . . . . . . . . . . . . . 17
3.2.1. Recommendations . . . . . . . . . . . . . . . . . . . 19
3.3. RDMA_NOMSG Call Messages . . . . . . . . . . . . . . . . 19
3.3.1. Recommendations . . . . . . . . . . . . . . . . . . . 20
3.4. RDMA_MSG Call with Position Zero Read Chunk . . . . . . . 20
3.4.1. Recommendations . . . . . . . . . . . . . . . . . . . 21
3.5. Padding Inline Content After A Chunk . . . . . . . . . . 21
3.5.1. Recommendations . . . . . . . . . . . . . . . . . . . 23
3.6. Write List XDR Roundup . . . . . . . . . . . . . . . . . 23
3.6.1. Recommendations . . . . . . . . . . . . . . . . . . . 24
3.7. Write List Error Cases . . . . . . . . . . . . . . . . . 25
3.7.1. Recommendations . . . . . . . . . . . . . . . . . . . 27
4. Operational Considerations . . . . . . . . . . . . . . . . . 27
4.1. Computing Request Buffer Requirements . . . . . . . . . . 27
4.1.1. Recommendations . . . . . . . . . . . . . . . . . . . 28
4.2. Default Inline Buffer Size . . . . . . . . . . . . . . . 28
4.2.1. Recommendations . . . . . . . . . . . . . . . . . . . 28
4.3. When To Use Reply Chunks . . . . . . . . . . . . . . . . 29
4.3.1. Recommendations . . . . . . . . . . . . . . . . . . . 29
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4.4. Computing Credit Values . . . . . . . . . . . . . . . . . 30
4.4.1. Recommendations . . . . . . . . . . . . . . . . . . . 30
4.5. Race Windows . . . . . . . . . . . . . . . . . . . . . . 30
4.5.1. Recommendations . . . . . . . . . . . . . . . . . . . 30
5. Pre-requisites for NFSv4 . . . . . . . . . . . . . . . . . . 31
5.1. Multiple RDMA-eligible Arguments and Results . . . . . . 31
5.1.1. Recommendations . . . . . . . . . . . . . . . . . . . 31
5.2. Bi-directional Operation . . . . . . . . . . . . . . . . 31
5.2.1. Recommendations . . . . . . . . . . . . . . . . . . . 31
5.3. Missing NFS Binding Specifications . . . . . . . . . . . 31
6. Requirements for Upper Layer Binding Specifications . . . . . 32
6.1. Organization Of Binding Specification Requirements . . . 32
6.1.1. Recommendations . . . . . . . . . . . . . . . . . . . 33
6.2. RDMA Eligibility . . . . . . . . . . . . . . . . . . . . 33
6.2.1. Recommendations . . . . . . . . . . . . . . . . . . . 33
6.3. Binding Specification Completion Assessment . . . . . . . 34
6.3.1. Recommendations . . . . . . . . . . . . . . . . . . . 34
7. Removal of Unimplemented Protocol Features . . . . . . . . . 34
7.1. Read-Read Transfer Model . . . . . . . . . . . . . . . . 34
7.1.1. Recommendations . . . . . . . . . . . . . . . . . . . 34
7.2. RDMA_MSGP . . . . . . . . . . . . . . . . . . . . . . . . 35
7.2.1. Recommendations . . . . . . . . . . . . . . . . . . . 35
8. Optional Additions To The Protocol . . . . . . . . . . . . . 35
8.1. Support For GSS-API With RPC-Over-RDMA . . . . . . . . . 35
8.2. Remote Invalidation . . . . . . . . . . . . . . . . . . . 36
8.2.1. Hardware Support . . . . . . . . . . . . . . . . . . 36
8.2.2. Avoiding Spurious Invalidation . . . . . . . . . . . 36
8.2.3. Invalidating Multiple R_keys . . . . . . . . . . . . 37
8.2.4. Invalidation Races . . . . . . . . . . . . . . . . . 37
8.2.5. Backward Compatibility . . . . . . . . . . . . . . . 37
8.2.6. Conclusion . . . . . . . . . . . . . . . . . . . . . 37
8.3. Work Cancellation . . . . . . . . . . . . . . . . . . . . 37
9. Security Considerations . . . . . . . . . . . . . . . . . . . 38
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 38
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
12.1. Normative References . . . . . . . . . . . . . . . . . . 39
12.2. Informative References . . . . . . . . . . . . . . . . . 40
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
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1.2. Purpose Of This Document
This document summarizes implementation experience with the RPC-over-
RDMA Version One protocol [RFC5666], and proposes improvements to the
protocol specification based on implementer experience, frequently-
asked questions, and interviews with a co-author of RFC 5666.
A key contribution of this document is to highlight areas of RFC 5666
where independent good faith readings could result in distinct
implementations that do not interoperate with each other. Correcting
these specification issues is critical: fresh implementations of RPC-
over-RDMA Version One continue to arise.
Recommendations are limited to the following areas:
o Repairing specification ambiguities
o Codifying successful implementation practices and conventions
o Clarifying the role of Upper Layer Binding specifications
o Exploring protocol enhancements that might be added without wire
behavior changes
1.3. Updating RFC 5666
This section is an unofficial summary of the nfsv4 Working Group
meeting held during IETF 92.
Several alternatives for updating RFC 5666 were discussed with the
RFC Editor and with the assembled members of the nfsv4 Working Group.
Among them were:
1. Filing individual errata for each issue.
2. Introducing an new RFC that updates but does not obsolete RFC
5666, but makes no change to the protocol.
3. Introducing an RFC 5666bis that replaces and thus obsoletes RFC
5666, but makes no change to the protocol.
4. Introducing a new RFC that specifies RPC-over-RDMA Version Two.
An additional possibility which is sometimes chosen by other Working
Groups would be to update RFC 5666 as it transitions from Proposed
Standard to Draft Standard.
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The overall preference observed during IETF 92 was to update and
obsolete RFC 5666, but retain full interoperability with current RPC-
over-RDMA Version One implementations by avoiding changes to on-the-
wire behavior (number 3 above). This eases the burden on
implementers, who can then reference a single specification of the
protocol. In addition, this alternative extends the life of the
current implementations in the field, which utilize RPC-over-RDMA
Version One effectively.
Subsequent discussion with the nfsv4 Working Group focused primarily
on resolving specification ambiguities that could result in
interoperability failure. A Version Two of RPC-over-RDMA, where
deeper changes can be made and new functionality introduced, was left
open for a later time. The priority is fixing issues with the
current Proposed Standard.
Recommendations in this document accepted by the nfsv4 Working Group
can be used as input when constructing an RFC 5666bis.
1.4. Scope Of This Document
This document does not specify a new Internet Protocol. It does not
propose changes to an existing Internet Protocol that are visible to
other implementations. It does not update a Proposed Standard, but
acts simply as a place to record specific areas that need attention.
Therefore the category of this document is Informational.
2. RPC-Over-RDMA Essentials
The following sections summarize the state of affairs defined in RFC
5666. This is a distillation of text from RFC 5666, dialog with a
co-author of RFC 5666, and implementer experience. The XDR
definitions are copied from RFC 5666 Section 4.3.
2.1. Arguments And Results
Like a local function call, every Remote Procedure Call (RPC)
operation has a set of one or more "arguments" and a set of one or
more "results." The calling context is not allowed to proceed until
the function's results are available. Unlike a local function call,
the called function is executed remotely rather than in the local
application's context.
A client endpoint, or "requester", serializes an RPC call's arguments
into a byte stream using XDR [RFC4506]. The XDR stream is conveyed
to a server endpoint via an RPC call message (sometimes referred to
as an "RPC request").
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The server endpoint, or "responder", deserializes the arguments and
processes the requested operation. It then serializes the
operation's results into an XDR byte stream. This stream is conveyed
back to the client endpoint via an RPC reply message. The client
deserializes the results and allows the original caller to proceed.
The remainder of this document assumes a working knowledge of XDR and
the RPC protocol [RFC5531].
2.2. Remote Direct Memory Access
An individual RPC argument or result may be very large. For example,
NFS READ and WRITE payloads are often 100KB or larger.
An RPC client system can be made more efficient if large RPC
arguments and results are transferred by a third party such as
intelligent network interface hardware. Remote Direct Memory Access
(RDMA) enables offloading data movement to avoid the negative
performance effects of using traditional host-based network
operations to move bulk data.
Another benefit of RDMA data transfer is that the host CPUs on both
transport endpoints are not involved. Data transfer on both the
sending and receiving endpoints is zero-touch. In particular, data
that is written to or read from a filesystem is opaque to the
transport layer, and thus can be transferred without any
serialization or other translation by the host CPU on either
endpoint.
RFC 5666 describes how to use only the Send, RDMA Read, and RDMA
Write operations described in [RFC5040] to move RPC calls and replies
between requesters and responders. The remainder of this document
assumes an understanding of RDMA and its primitives.
Because RDMA Read and Write operations work most efficiently with
large payloads, RPC-over-RDMA Version One moves RPCs with large
payloads differently than RPCs with small payloads.
2.2.1. Small Data Transfers
A local endpoint transfers data into small unadvertised buffers on a
remote endpoint using Send operations. Each transfer behaves like a
reliable datagram send operation.
This transfer mode is utilized to convey small RPC operations and
advertisements of buffer coordinates for large data transfers (see
below). The latency of Send operations is significantly lower than
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traditional network transfers, but the size of these operations is
typically limited.
2.2.2. Large Data Transfers
A local endpoint tags memory areas to be involved in data transfers,
then advertises the coordinates of those areas to a remote endpoint.
The remote endpoint transfers data into or out of those areas using
RDMA Read and Write operations.
Finally the remote endpoint signals that its work is done, and the
local endpoint ensures remote access to the memory area is no longer
allowed.
This transfer mode is utilized to convey large whole RPCs or single
large arguments or results.
2.3. Transfer Models
A "transfer model" describes which endpoint is responsible for
performing RDMA Read and Write operations. The opposite endpoint
must expose part or all of its memory, and advertise the coordinates
of that memory.
2.3.1. Read-Read
Requesters expose their memory to the responder, and the responder
exposes its memory to requesters. The responder employs RDMA Read
operations to convey RPC arguments or whole RPC calls. Requesters
employ RDMA Read operations to convey RPC results or whole RPC
relies.
Although this model is specified in RFC 5666, no current RPC-over-
RDMA Version One implementation uses the Read-Read transfer model.
2.3.2. Write-Write
Requesters expose their memory to the responder, and the responder
exposes its memory to requesters. Requesters employ RDMA Write
operations to convey RPC arguments or whole RPC calls. The responder
employs RDMA Write operations to convey RPC results or whole RPC
relies.
The Write-Write transfer model is used by a few other storage
protocols, but is not considered in RFC 5666.
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2.3.3. Read-Write
Requesters expose their memory to the responder, but the responder
does not expose its memory. The responder employs RDMA Read
operations to convey RPC arguments or whole RPC calls. The responder
employs RDMA Write operations to convey RPC results or whole RPC
relies.
This model is specified in RFC 5666. All known RPC-over-RDMA Version
One implementations employ this model. For clarity, the remainder of
this document considers only the Read-Write transfer model.
2.4. Upper Layer Binding Specifications
RFC 5666 provides a framework for conveying RPC requests and replies
on RDMA transports. By itself this is insufficient to enable an RPC
program, referred to as an "Upper Layer Protocol" or ULP, to operate
over an RDMA transport.
Arguments and results come in different sizes and have different
serialization requirements, all depending on the Upper Layer
Protocol. Some arguments and results are appropriate for RDMA
transfer, while others are not. Thus RFC 5666 requires additional
separate specifications that describe how each ULP may use RDMA. The
set of requirements for a ULP to use an RDMA transport is known as an
"Upper Layer Binding" specification, or ULB.
An Upper Layer's ULB states which RPC arguments and results in the
RPC program are permitted to be transferred by RDMA Read and Write.
These are sometimes referred to as "RDMA-eligible." These
restrictions do not apply when a whole RPC call or reply is
transmitted via an RDMA operation.
A ULB is required for each RPC program and version tuple that is
interested in operating on an RDMA transport. A ULB may be part of
another specification, or it may be a stand-alone document, similar
to [RFC5667].
2.5. On-The-Wire Protocol
2.5.1. Inline Operation
Each RPC call or reply message conveyed on an RDMA transport starts
with an RPC-over-RDMA header. A requester uses a Send operation to
convey the RPC-over-RDMA header to a responder. A responder does
likewise to convey RPC replies back to a requester. The message
contents sent via Send, including an RPC-over-RDMA header and
possibly an RPC message proper, are referred to as "inline content."
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The RPC-over-RDMA header starts with three uint32 fields:
<CODE BEGINS>
struct rdma_msg {
uint32 rdma_xid; /* Mirrors the RPC header xid */
uint32 rdma_vers; /* Version of this protocol */
uint32 rdma_credit; /* Buffers requested/granted */
rdma_body rdma_body;
};
<CODE ENDS>
Following these three fields is a union:
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<CODE BEGINS>
enum rdma_proc {
RDMA_MSG=0, /* An RPC call or reply msg */
RDMA_NOMSG=1, /* An RPC call or reply msg -
separate body */
. . .
RDMA_ERROR=4 /* An RPC RDMA encoding error */
};
union rdma_body switch (rdma_proc proc) {
case RDMA_MSG:
rpc_rdma_header rdma_msg;
case RDMA_NOMSG:
rpc_rdma_header_nomsg rdma_nomsg;
. . .
case RDMA_ERROR:
rpc_rdma_error rdma_error;
};
struct rpc_rdma_header {
struct xdr_read_list *rdma_reads;
struct xdr_write_list *rdma_writes;
struct xdr_write_chunk *rdma_reply;
/* rpc body follows */
};
struct rpc_rdma_header_nomsg {
struct xdr_read_list *rdma_reads;
struct xdr_write_list *rdma_writes;
struct xdr_write_chunk *rdma_reply;
};
<CODE ENDS>
In either the RDMA_MSG or RDMA_NOMSG case, the RPC-over-RDMA header
may advertise memory coordinates to be used for RDMA data transfers
associated with this RPC.
The difference between these two cases is whether or not the
traditional RPC header itself is included in this Send operation
(RDMA_MSG), or not (RDMA_NOMSG). In the former case, the RPC header
follows immediately after the rdma_reply field. In the latter case,
the RPC header is transfered via another mechanism (typically a
separate RDMA Read operation).
A requester may use either type of message to send an RPC call
message, depending on the requirements of the RPC call message being
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conveyed. A responder may use RDMA_NOMSG only when the requester
provides a Reply chunk (see Section 4.3). A responder is free to use
RDMA_MSG instead in that case, depending on the requirements of the
RPC reply message.
2.5.2. RDMA Segment
An "RDMA segment", or just "segment", contains the co-ordinates of a
contiguous memory region that is to be conveyed via an RDMA Read or
RDMA Write operation.
A segment is advertised in an RPC-over-RDMA header to enable the
receiving endpoint to drive subsequent RDMA access of the data in
that memory region. The RPC-over-RDMA Version One XDR represents an
RDMA segment with the xdr_rdma_segment struct:
<CODE BEGINS>
struct xdr_rdma_segment {
uint32 handle;
uint32 length;
uint64 offset;
};
<CODE ENDS>
See [RFC5040] for a discussion of what the content of these fields
means.
2.5.3. Read Chunk
One or more "read chunks" are used to advertise the coordinates of an
RPC argument to be transferred via an RDMA Read operation. Each read
chunk is represented by the xdr_read_chunk struct:
<CODE BEGINS>
struct xdr_read_chunk {
uint32 position;
struct xdr_rdma_segment target;
};
<CODE ENDS>
A read chunk is one RDMA segment with a Position field. The Position
field indicates the location in an XDR stream where the argument's
data would appear if it were being transferred inline.
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A single RPC argument might be contained in one contiguous memory
region. That RPC argument can be represented by a single read chunk.
Alternately, a single RPC argument might reside in multiple
discontiguous memory regions. Since the memory regions are not
contiguous, each region is represented by a single read chunk in a
list of chunks. The definition of Position in RFC 5666 Section 3.4
implies this by saying "all chunks belonging to a single RPC
argument... will have the same position."
Thus all read chunks that belong to the same RPC argument have the
same value in their Position field, and are read in list order into
memory regions on the responder. This enables gathering RPC argument
data from multiple buffers on the requester.
2.5.4. Write Chunk
A "Write chunk" conveys an RPC result object using one or more RDMA
Write operations.
Each write chunk is an array of RDMA segments. One RDMA-eligible RPC
result is always conveyed in a single write chunk. This is unlike an
RDMA-eligible RPC argument, which may be conveyed in more than one
read chunk.
A write chunk is represented by the xdr_write_chunk struct:
<CODE BEGINS>
struct xdr_write_chunk {
struct xdr_rdma_segment target<>;
};
<CODE ENDS>
These segments are written in array order into memory regions on the
requester This enables scattering an RPC result's data into multiple
buffers on the requester.
A requester provides a write chunk as a receptacle for an RPC result.
Typically the exact size of the result cannot be predicted before the
responder has formed its reply. Thus the requester must provide
enough space in the write chunk for the largest result the responder
might generate for this RPC operation. The responder updates the
size of each segment in the Write chunk when it returns the Write
list to the requester via a matching RPC reply message.
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Because the requester must pre-allocate the area in which the
responder writes the result before the responder has formed the
reply, giving a position and size to the data, the requester cannot
know the XDR position of the reply object. Thus write chunks do not
have a Position field.
2.5.5. Read List
Each RPC-over-RDMA Version One call has one "Read list," provided by
the requester. This is a list of zero or more RDMA segments with
Position values that make up all the RPC arguments in this RPC
request to be conveyed via RDMA Read operations.
A Read list is represented by the xdr_read_list struct:
<CODE BEGINS>
struct xdr_read_list {
struct xdr_read_chunk entry;
struct xdr_read_list *next;
};
<CODE ENDS>
The Read list may be empty if the RPC call has no RPC arguments that
are RDMA-eligible.
2.5.6. Write List
Each RPC-over-RDMA Version One call has one "Write list," provided by
the requester. This is a list of zero or more RDMA segment arrays
that will catch the RPC results in this RPC request to be conveyed
via RDMA Write operations.
A Write list is represented by the xdr_write_list struct:
<CODE BEGINS>
struct xdr_write_list {
struct xdr_write_chunk entry;
struct xdr_write_list *next;
};
<CODE ENDS>
Note that this looks similar to a Read list, but because an
xdr_write_chunk is an array and not an RDMA segment, the two data
structures are not the same.
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The Write list may be empty if there are no RPC results which are
RDMA-eligible.
2.5.7. Position Zero Read Chunk
A requester may use a "Position Zero read chunk" to convey most or
all of an entire RPC call, rather than including the RPC call message
inline. A Position Zero read chunk is necessary if the RPC call
message is too large to fit inline. RFC 5666 Section 5.1 defines the
operation of a "Position Zero read chunk."
To support gathering a large RPC call message from multiple locations
on the requester, a Position Zero read chunk may be comprised of more
than one xdr_read_chunk. Each read chunk that belongs to the
Position Zero read chunk has the value zero in its Position field.
2.5.8. Reply Chunk
Each RPC-over-RDMA Version One call may have one "Reply chunk,"
provided by the requester. A Reply chunk is a write chunk, thus it
is an array of one or more RDMA segments. This enables a requester
to control where the responder scatters the parts of the RPC reply
message. Typically there is only one segment in a Reply chunk.
A requester provides the Reply chunk whenever it predicts the
responder's reply cannot fit inline. It may choose to provide the
Reply chunk even when the responder can return only a small reply. A
responder may use a "Reply chunk" to convey most or all of an entire
RPC reply, rather than including the RPC reply message inline.
3. Specification Issues
3.1. XDR Clarifications
Even seasoned NFS/RDMA implementers have had difficulty agreeing on
precisely what a "chunk" is, and had challenges distinguishing the
structure of the Read list from structure of the Write list.
On occasion, the text of RFC 5666 uses the term "chunk" to represent
either read chunks or write chunks, even though these are different
data types and have different semantics.
For example, RFC 5666 Section 3.4 uses the term "chunk list entry"
even though the discussion is referring to an array element. It
implies all chunk types have a Position field, even though only read
chunks have this field.
Near the end of Section 3.4, it says:
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Therefore, read chunks are encoded into a read chunk list as a
single array, with each entry tagged by its (known) size and its
argument's or result's position in the XDR stream.
The Read list is not an XDR array, it is always an XDR list. A Write
chunk is an XDR array.
RFC 5666 Section 3.7, third paragraph uses the terms "chunked
element" and "chunk segment." Neither term is defined or used
anywhere else. The fourth paragraph refers to a "sequence of chunks"
but likely means a sequence of RDMA segments.
The Read list is typically used for Upper Layer WRITE operations such
as NFS WRITE, while the Write list is typically used for Upper Layer
READ operations such as NFS READ. If the Read-Read transfer model is
removed from RFC 5666bis, it would be less confusing to readers of
Upper Layer Binding specifications to call the Read list the Argument
list, and call the Write list the Result list.
The XDR definition for a read chunk is an RDMA segment with a
position field. It is implied in RFC 5666 Section 3.4 that multiple
xdr_read_chunk objects can make up a single RPC argument object if
they share the same Position in the XDR stream. Some implementations
depend on using multiple RDMA segments in the same XDR Position,
particularly for sending Position Zero read chunks efficiently by
gathering an RPC call message from multiple discontiguous memory
locations. Other implementations do not support sending or receiving
multiple Read chunks with the same Position.
Upper Layer Binding documents may limit the number of Read list
entries allowed in a particular operation. In that case, the ULB is
not restricting the total number of read chunks in the list, but
rather the total number of distinct Positions that appear in the
list.
The XDR definition for a write chunk is an array of segments. One
xdr_write_chunk represents one RPC result object. An RPC argument is
represented by one or more read chunks, but an RPC result is always
represented by a single write chunk.
The Write list is especially confusing because it is a list of arrays
of RDMA segments, rather than a simple list of xdr_read_chunk
objects. What is referred to as a Read list entry often means one
xdr_read_chunk, or one segment. That segment can be either a portion
of or a whole RPC argument. A Write list entry is an array, and is
always a whole RPC result.
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An Upper Layer Binding may limit the number of chunks in a Write list
allowed for a particular operation. That strictly limits the number
of Write list entries.
Not having a firm one-to-one correspondence between read chunks and
RPC arguments is sometimes awkward. The two chunk types should be
more symmetrical to avoid confusion, although that might be difficult
to pull off without altering the RPC-over-RDMA Version One XDR
definition. As we will see later, the XDR roundup rules also appear
to apply asymmetrically to read chunks and write chunks.
Implementers have been aided by the ASCII art block comments in the
Linux kernel in net/sunrpc/xprtrdma/rpcrdma.c, excerpted here. This
diagram shows exactly how the Read list and Write list are
constructed in an XDR stream.
<CODE BEGINS>
/*
* Encoding key for single-list chunks
* (HLOO = Handle32 Length32 Offset64):
*
* Read chunklist (a linked list):
* N elements, position P (same P for all chunks of same arg!):
* 1 - PHLOO - 1 - PHLOO - ... - 1 - PHLOO - 0
*
* Write chunklist (a list of (one) counted array):
* N elements:
* 1 - N - HLOO - HLOO - ... - HLOO - 0
*
* Reply chunk (a counted array):
* N elements:
* 1 - N - HLOO - HLOO - ... - HLOO
*/
<CODE ENDS>
3.1.1. Recommendations
To aid in understanding, RFC 5666bis should include a glossary that
explains and distinguishes the various elements in the protocol.
Upper Layer Binding specifications may also refer to these terms.
RFC 5666bis should utilize and capitalize these glossary terms
consistently.
RFC 5666bis should introduce additional diagrams that supplement the
XDR definition in RFC 5666 Section 4.3. RFC 5666bis should explain
the structure of the XDR and how it is used. RFC 5666bis should
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contain an explicit but brief rationalization for the structural
differences between the Read list and the Write list.
RFC 5666bis should use a consistent naming convention for all XDR
definitions. For example, all structures and union names should use
an "rpc_rdma1_" prefix.
To address conflation of a read chunk that is a single xdr_read_chunk
and a read chunk that is a list of xdr_read_chunk elements with
identical Position field values, the following specification changes
should be made:
o Rename the xdr_read_chunk XDR object as rpc_rdma1_read_segment.
o Define a "read chunk" as an ordered list of rpc_rdma1_read_segment
objects that have identical Position values.
o Define the "Read list" as a list of zero or more read chunks,
expressed as an ordered list of rpc_rdma1_read_segment objects
whose Position value may vary.
With this change, there would no longer be a simple XDR object that
explicitly represents a read chunk. A read chunk and a write chunk
are now equivalent objects: One read chunk will always map to a
single RPC argument, just like a write chunk always maps to a single
RPC result. All discussion should take care to use the term
"segment" and "read segment" instead of the term "read chunk" where
appropriate.
As a clean up, RFC 5666bis should remove the rpc_rdma_header_nomsg
struct, and use the rpc_rdma_header struct in its place. Since
rpc_rdma_header does not comprise the entire RPC-over-RDMA header, it
should be renamed rpc_rdma1_chunks to avoid confusion.
XDR definitions should be enclosed in CODE BEGINS and CODE ENDS
delimiters. An appropriate copyright block should accompany the XDR
definitions in RFC 5666bis. An XDR extraction shell script should be
provided in the text.
3.2. The Position Zero Read Chunk
RFC 5666 Section 5.1 defines the operation of the Position Zero read
chunk. A requester uses the Position Zero read chunk in place of
inline content. A requester is required to use the Position Zero
read chunk when the total size of an RPC call exceeds the size of the
responder's receive buffers and the ULB prohibits the use of RDMA for
large RPC arguments. The requester conveys the co-ordinates of the
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Position Zero read chunk with a Send operation, then the responder
uses an RDMA Read operation to pull the RPC call message.
RFC 5666 Section 3.4 says:
Semantically speaking, the protocol has no restriction regarding
data types that may or may not be represented by a read or write
chunk. In practice however, efficiency considerations lead to the
conclusion that certain data types are not generally "chunkable".
Typically, only those opaque and aggregate data types that may
attain substantial size are considered to be eligible. With
today's hardware, this size may be a kilobyte or more. However,
any object MAY be chosen for chunking in any given message.
The eligibility of XDR data items to be candidates for being moved
as data chunks (as opposed to being marshaled inline) is not
specified by the RPC-over-RDMA protocol. Chunk eligibility
criteria MUST be determined by each upper-layer in order to
provide for an interoperable specification.
The intention of this text is to spell out that RDMA eligibility
applies only to individual arguments and results, and RDMA
eligibility criteria is determined by a separate specification, and
not in RFC 5666.
The Position Zero read chunk is an exception to both of these
guidelines. The Position Zero read chunk, by virtue of the fact that
it typically conveys an entire RPC call message, may contain multiple
arguments, independent of whether any particular argument in the RPC
call is RDMA-eligible.
Unlike the read chunks described in the RFC 5666 excerpt above, the
content of a Position Zero read chunk is typically marshaled and
copied on both ends of the transport, negating the benefit of RDMA
data transfer. In particular, the Position Zero read chunk is not
for conveying performance critical Upper Layer operations.
Thus the requirements for what may or may not appear in the Position
Zero read chunk are indeed specified by RFC 5666, in contradiction to
the second paragraph quoted above. Upper Layer Binding
specifications may have something to say about what may appear in the
Position Zero read chunk, but the basic definition of Position Zero
should be made clear in RFC 5666bis as distinct from a read chunk
whose Position field is non-zero.
Because a read chunk is defined as one RDMA segment with a Position
field, at least one implementation allows only a single chunk segment
in Position zero read chunks. This is a problem for two reasons:
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o Some RPCs are constructed in multiple non-contiguous buffers.
Allowing only one read segment in Position Zero would mean a
single large contiguous buffer would be have to be allocated and
registered, and then the components of the XDR stream would have
to be copied into that buffer.
o Some requesters might not be able to register memory regions
larger than the platform's physical page size. Allowing only one
read segment in Position Zero would limit the maximum size of RPC-
over-RDMA messages to a single page. Allowing multiple read
segments means the message size can be as large as the maximum
number of read chunks that can be sent in an RPC-over-RDMA header.
RFC 5666 does not limit the number of read segments in a read chunk,
nor does it limit the number of chunks that can appear in the Read
list. The Position Zero read chunk, despite its name, is not limited
to a single xdr_read_chunk.
3.2.1. Recommendations
RFC 5666bis should state that the guidelines in RFC 5666 Section 3.4
apply only to RDMA_MSG type calls. When the Position Zero read chunk
is introduced in RFC 5666 Section 5.1, enumerate the differences
between it and the read chunks previously described in RFC 5666
Section 3.4.
RFC 5666bis should describe what restrictions an Upper Layer Binding
may make on Position Zero read chunks.
3.3. RDMA_NOMSG Call Messages
The second paragraph of RFC 5667 Section 4 says, in reference to
NFSv2 and NFSv3 WRITE and SYMLINK operations:
. . . a single RDMA Read list entry MAY be posted by the client to
supply the opaque file data for a WRITE request or the pathname
for a SYMLINK request. The server MUST ignore any Read list for
other NFS procedures, as well as additional Read list entries
beyond the first in the list.
However, large non-write NFS operations are conveyed via a Read list
containing at least a Position Zero read chunk. Strictly speaking,
the above requirement means large non-write NFS operations may never
be conveyed because the responder MUST ignore the read chunk in such
requests.
It is likely the authors of RFC 5667 intended this limit to apply
only to RDMA_MSG type calls. If that is true, however, an NFS
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implementation could legally skirt the stated restriction simply by
using an RDMA_NOMSG type call that conveys both a Position Zero and a
non-zero position read chunk to send a non-write NFS operation.
Unless either RFC 5666 or the protocol's Upper Layer Binding
explicitly prohibits it, allowing a read chunk in a non-zero Position
in an RDMA_NOMSG type call means an Upper Layer Protocol may ignore
Binding requirements like the above.
Typically there is no benefit to allowing multiple read chunks for
RDMA_NOMSG type calls. Any non-zero Position read segments can
always be conveyed as part of the Position Zero read chunk.
However, there is a class of RPC operations where RDMA_NOMSG with
multiple read chunks is useful: when the body of an RPC call message
is larger than the inline buffer size, even after bulk payload has
been placed in read chunks.
A similar discussion applies to RDMA_NOMSG replies with large reply
bodies and RDMA-eligible results. Such replies would use both the
Write list and the Reply chunk simultaneously. However, write chunks
do not have Position fields. It remains to be seen whether this is
enough to enable requesters to re-assemble generic RPC replies
correctly.
3.3.1. Recommendations
RFC 5666bis should continue to allow RDMA_NOMSG type calls with
additional read chunks. The rules about RDMA-eligibility in RFC
5666bis should discuss when the use of this construction is
beneficial, and when it should be avoided.
Authors of Upper Layer Bindings should be warned about ignoring these
cases. RPC 5666bis should provide a default behavior that applies
when Upper Layer Bindings omit this discussion.
3.4. RDMA_MSG Call with Position Zero Read Chunk
An RPC header starts at XDR stream offset zero. The first item in
the header of both RPC calls and RPC replies is the XID field
[RFC5531]. RFC 5666 Section 4.1 says:
A header of message type RDMA_MSG or RDMA_MSGP MUST be followed by
the RPC call or RPC reply message body, beginning with the XID.
This is a strong implication that the RPC header in an RDMA_MSG type
message starts at XDR stream offset zero.
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An RDMA_MSG type call message includes the RPC header and zero or
more read chunks. Recall the definition of a read chunk as a list of
read segments whose Position field contains the same value. The
value of the Position field determines where the read chunk appears
in the XDR stream that comprises an RPC call message.
A Position Zero read chunk, therefore, starts at XDR stream offset
zero, just like RPC header does. In an RDMA_NOMSG type call message,
which does not include an RPC header, a Position Zero read chunk
conveys the RPC header.
There is no prohibition in RFC 5666 against an RDMA_MSG type call
messsage with a Position Zero read chunk. However, it's not clear
how a responder should interpret such a message. RFC 5666 requires
the RPC header to start at XDR stream offset zero, but there is a
Position Zero read chunk, which also starts at XDR stream offset
zero.
3.4.1. Recommendations
RPC 5666bis should clearly define what is meant by an XDR stream.
RFC 5666bis should state that XDR stream Position is measured
relative to the start of the RPC header, which is the first byte of
the header's XID field.
RFC 5666bis should prohibit requesters from providing a Position Zero
read chunk in RDMA_MSG type calls. Likewise, RFC 5666bis should
prohibit responders from utilizing a Reply chunk in RDMA_MSG type
replies.
The diagrams in RFC 5666 Section 3.8 which number chunks starting
with 1 are confusing and should be revised. Numbering chunks this
way is not natural to the way read chunks and write chunks work.
3.5. Padding Inline Content After A Chunk
To help clarify the discussion in this section, the term "read chunk"
here always means the new definition where one or more read segments
that have identical values in their Position fields represents
exactly one RPC argument.
A read chunk conveys a large RPC argument via one or more RDMA
transfers. For instance, the data payload of an NFS WRITE operation
may be be transferred using a read chunk [RFC5667].
NFSv3 WRITE operations place the data payload at the end of an RPC
call message [RFC1813]. The RPC call's XDR stream starts in an
inline buffer, continues in a read chunk, then ends there.
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An NFSv4 WRITE operation may occur as a middle operation in an NFSv4
COMPOUND [RFC5661]. The read chunk containing the data payload
argument of the WRITE operation might finish before the RPC call's
XDR stream does. In this case, the RPC call's XDR stream starts in
an inline buffer, continues in the Read list, then finishes back in
the inline buffer.
The length of a chunk is the sum of the lengths of the segments that
make up that chunk. The data payload in a chunk may have a length
that is not evenly divisible by four. One or more of the segments
may have an unaligned length.
RFC 5666 Section 3.7 describes how to manage XDR roundup in a read
chunk when its length is not XDR-aligned. The sender is not required
to send the extra pad bytes at the end of a chunk because a) the
receiver never references their content, therefore it is wasteful to
transmit them, and b) each read chunk has a Position field and length
that determines exactly where that chunk starts and ends in the XDR
stream.
A question arises, however, when considering where the next argument
after a read chunk should appear. XDR requires each argument in an
RPC call to begin on 4-byte alignment [RFC4506]. But a read chunk's
XDR padding is optional (see above). The next read chunk's position
field determines where it is placed in the XDR stream. However
inline content following a read chunk does not have a Position field
to guide the receiver in the reassembly of the RPC call message.
Paragraph 4 of RFC 5666 Section 3.7 says:
When roundup is present at the end of a sequence of chunks, the
length of the sequence will terminate it at a non-4-byte XDR
position. When the receiver proceeds to decode the remaining part
of the XDR stream, it inspects the XDR position indicated by the
next chunk. Because this position will not match (else roundup
would not have occurred), the receiver decoding will fall back to
inspecting the remaining inline portion. If in turn, no data
remains to be decoded from the inline portion, then the receiver
MUST conclude that roundup is present, and therefore it advances
the XDR decode position to that indicated by the next chunk (if
any). In this way, roundup is passed without ever actually
transferring additional XDR bytes.
This paragraph adequately describes XDR padding requirements when a
read chunk is followed by another read chunk. But it leaves open any
requirements for XDR padding and alignment when a read chunk is
followed in the XDR stream by more inline content.
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The correct answer is that following a read chunk of an unaligned
length, if the next argument in the XDR stream is in the inline
buffer, it must begin on a 4-byte boundary in that buffer, even when
XDR padding is not included in the preceding read chunk. This is
because the object that follows a read chunk must always start on an
XDR alignment boundary.
Furthermore, the XDR pad for the preceding read chunk cannot appear
in the inline content, even if it was also not included in the chunk
itself. This is because the RPC argument that preceded the read
chunk will have been padded to 4-byte alignment. The next position
in the inline buffer will already be on a 4-byte boundary.
3.5.1. Recommendations
State the above requirement in RFC 5666bis in its equivalent of RFC
5666 Section 3.7. When a responder forms a reply, the same
restriction applies to inline content interleaved with write chunks.
A good generic rule is that all RPC objects in every call or reply
message must start on an XDR alignment boundary. This has
implications for the values allowed in read chunk Position fields,
for how XDR roundup works for chunks, and for how RPC objects are
placed in inline buffers. XDR alignment in inline buffers is always
relative to Position Zero (or, where the RPC header starts).
3.6. Write List XDR Roundup
The final paragraph of RFC 5666 Section 3.7 says this:
For RDMA Write Chunks, a simpler encoding method applies. Again,
roundup bytes are not transferred, instead the chunk length sent
to the receiver in the reply is simply increased to include any
roundup.
A responder should never write XDR pad bytes, as the requester's
upper layers does not reference them. However, for the chunk length
to be rounded up as described, the requester must provide adequate
extra space in the chunk for the XDR pad. A requester can provide
space for the XDR pad using one of two approaches:
1. It can extend the last segment in the chunk.
2. It can provide another segment after the segments that receive
RDMA Write payloads.
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Case 1 is adequate when there is no danger that the responder's RDMA
Write operations will overwrite existing data on the requester in
buffers following the advertised receive buffers.
In zero-copy scenarios, an extra segment must be provided separately
to avoid overwriting existing data (case 2). In cases where live
data follows the area where the responder writes the data payload, an
extra registration is needed for just a handful of bytes of no value.
Registering the extra buffer is a needless cost. It would be more
efficient if the XDR pad at the end of a write chunk were treated the
same as it is for read chunks. Because every RPC result object must
begin on an XDR alignment boundary, the object following the write
chunk in the reply's XDR stream must begin on an XDR alignment
boundary. There should be no need for a XDR pad to be present for
the receiver to re-assemble the RPC reply's XDR stream correctly.
Unfortunately at least one server implementation relies on the
existence of that extra buffer, even though it does not write to it.
Another server implementation does not rely on it (operation proceeds
if it is missing) but when it is present, this server does write
zeroes to it.
Therefore the extra buffer for a write chunk's XDR pad, either as a
separate segment, or as an extension of the segment that represents
the data payload buffer, must remain for now.
Note that because the Reply chunk is a write chunk, these roundup
rules apply to it as well. However, a requester typically provides a
single contiguous buffer for whole replies, which consist of XDR
encoded content. A separate tail buffer to catch an XDR pad is
unlikely to be needed.
3.6.1. Recommendations
RFC 5666bis should provide a discussion of the requirements around
write chunk roundup, with examples. The discussion should be
separate from the discussion of read chunk roundup.
Explicit RFC2119-style interoperability requirements should be
provided in the text. For example, the requester MUST provide buffer
space for XDR roundup of write chunks, and the responder SHOULD NOT
write into that buffer.
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3.7. Write List Error Cases
RFC 5666 Section 3.6 says:
When a write chunk list is provided for the results of the RPC
call, the RPC server MUST provide any corresponding data via RDMA
Write to the memory referenced in the chunk list entries.
This requires the responder to use the Write list when it is
provided. Another way to say it is a responder is not permitted to
return bulk data inline or in the reply chunk when the requester has
provided a Write list.
This requirement is less clear when it comes to situations where a
particular RPC reply is allowed to use a provided Write list, but
does not have a bulk data payload to return. For example, RFC 5667
Section 4 permits requester to provide a Write list for NFS READ
operations. However, NFSv3 READ operations have a union reply
[RFC1813]:
<CODE BEGINS>
struct READ3resok {
post_op_attr file_attributes;
count3 count;
bool eof;
opaque data<>;
};
struct READ3resfail {
post_op_attr file_attributes;
};
union READ3res switch (nfsstat3 status) {
case NFS3_OK:
READ3resok resok;
default:
READ3resfail resfail;
};
<CODE ENDS>
The arm of the READ3res union which is used when a read error occurs
does not have a bulk data argument. When an NFS READ operation
fails, no data is returned.
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RFC 5666 does not prescribe how a responder should behave when the
result object for which the Write list is provided does not appear in
the reply. RFC 5666 Section 3.4 says:
Individual write chunk list elements MAY thereby result in being
partially or fully filled, or in fact not being filled at all.
Unused write chunks, or unused bytes in write chunk buffer lists,
are not returned as results, and their memory is returned to the
upper layer as part of RPC completion.
It also says:
The RPC reply conveys this by returning the write chunk list to
the client with the lengths rewritten to match the actual
transfer.
The disposition of the advertised write buffers is therefore clear.
The requirements for how the Write list must appear in an RPC reply
are somewhat less than clear.
Here we are concerned with two cases:
o When a result consumes fewer RDMA segments than the requester
provided in the Write chunk for that result, what values are
provided for the chunk's segment count, and the lengths of the
unused segments
o When a result is not used (say, the reply uses the arm of an XDR
union that does not contain the result corresponding to a Write
chunk provided for that result), what values are provided for the
chunk's segment count, and the lengths of the unused segments
The language above suggests the proper value for the Write chunk's
segment count is always the same value that the requester sent, even
when the chunk is not used in the reply. The proper value for the
length of an unused segment in a Write chunk is always zero.
Inspection of one existing server implementation shows that when an
NFS READ operation fails, the returned Write list contains one entry:
a chunk array containing zero elements. Another server
implementation returns the original Write list chunk in this case.
In either case, requesters appear to ignore the Write list when no
bulk data payload is expected. Thus it appears that, currently,
responders may put whatever they like in the Write list.
In the future, RPC-over-RDMA Version One will have to handle RPC
replies where multiple Write list entries are available but the
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responder has a choice about which result objects to return as bulk
reply data. The arguments and results of an NFSv4 COMPOUND are a
switched union, and some of the operations in a compound (such as
READ, whose data payload reply is RDMA-eligible) also use a switched
union.
For example, combining several READ operations in an NFSv4 COMPOUND
might be problematic (if it weren't for the requirement that the
entire compound should fail if just one operation in the compound
fails).
3.7.1. Recommendations
RFC 5666bis should explicitly discuss responder behavior when an RPC
reply does not need to use a Write list entry provided by a
requester. This is generic behavior, independent of any Upper Layer
Binding. The explanation can be partially or wholly copied from RFC
5667 Section 5's discussion of NFSv4 COMPOUND.
A number of places in RFC 5666 Section 3.6 hint at how a responder
behaves when it is to return data that does not use every byte of
every provided Write chunk segment. RFC 5666bis should state
specific requirements about how a responder should form the Write
list in RPC replies, and/or it should explicitly require requesters
to ignore the Write list in these cases. A good quality requester
implementation would save the Write list and use that saved copy to
invalidate the written memory region upon RPC completion. RFC
5666bis should require that the responder not alter the count of
segments in the Write chunk. One or more explicit examples should be
provided in RFC 5666bis.
RFC 5666bis should provide clear instructions on how Upper Layer
Bindings are to be written to take care of switched unions.
4. Operational Considerations
4.1. Computing Request Buffer Requirements
The size maximum of a single Send operation includes both the RPC-
over-RDMA header and the RPC header. Combined, those two headers
must not exceed the size of one receive buffer.
Senders often construct the RPC-over-RDMA header and the RPC call or
reply message in separate buffers, then combine them via an iovec
into a single Send. This does not mean each element of that iovec
can be as large as the inline threshold.
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An HCA or RNIC may have a small limit on the size of a registered
memory region. In that case, each argument or result may be
comprised of many chunk segments.
This has implications for the size of the Read and Write lists, which
take up a variable amount of space in the RPC-over-RDMA header. The
sum of the size of the RPC-over-RDMA header, including the Read and
Write lists, and the size of the RPC header must not exceed the
inline threshold. This limits the maximum Upper Layer payload size.
4.1.1. Recommendations
RFC 5666bis should provide implementation guidance on how the inline
threshold (the maximum send size) is computed.
4.2. Default Inline Buffer Size
Section 6 of RFC 5666 specifies an out-of-band protocol that allows
an endpoint to discover a peer endpoint's receive buffer size, to
avoid overrunning the receiving buffer, causing a connection loss.
Not all RPC-over-RDMA Version One implementations also implement CCP,
as it is optional. Given the importance of knowing the receiving
end's receive buffer size, there should be some way that a sender can
choose a size that is guaranteed to work with no CCP interaction.
RFC 5666 Section 6.1 describes a 1KB receive buffer limit for the
first operation on a connection with an unfamiliar responder. In the
absence of CCP, the client cannot discover that responder's true
limit without risking the loss of the transport connection.
4.2.1. Recommendations
RFC 5666bis should specify a fixed send/receive buffer size as part
of the RPC-over-RDMA Version One protocol, to use when CCP is not
available. For example, the following could be added to the RFC
5666bis equivalent of RFC 5666 Section 6.1: "In the absence of CCP,
requesters and responders MUST assume 1KB receive buffers for all
Send operations."
It should be safe for Upper Layer Binding specifications to provide a
different default inline threshold. Care must be taken when an
endpoint is associated with multiple RPC programs that have different
default thresholds.
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4.3. When To Use Reply Chunks
RFC 5666 Section 3.6 says:
When a write chunk list is provided for the results of the RPC
call, the RPC server MUST provide any corresponding data via RDMA
Write to the memory referenced in the chunk list entries.
The Reply chunk is a write chunk (a degenerate write chunk list). It
is not clear whether the authors intended this requirement to apply
to the Reply chunk. Some server implementations regard the Reply
chunk as optional.
Requesters may always provide a Reply chunk, at the cost of
registering memory the server may choose not to use. Or a client may
choose not to provide a Reply chunk when it believes there is no
possibility the server will overrun the client's receive buffer when
returning the RPC reply.
A server may always use a provided Reply chunk, even when it is more
efficient to convey an RPC reply inline (for instance, if an RPC
reply is very small). Or a server may choose to ignore the provided
Reply chunk when it believes there is no possibility the RPC reply
can overrun the client's receive buffer.
The choice of when to provide or utilize a reply chunk depends on
whether the sender believes the RPC message will fit entirely within
the inline buffer.
Section 3.6 of RFC 5667 says a server MUST use a Write list provided
by a client. RFC 5666bis might prescribe that if the client provides
a Reply chunk, the server MUST use it, as the client is telling the
server that it believes the expected RPC reply may not fit in its
receive buffer. That way the server cannot overrun client's receive
buffer by choosing to Send an intermediate-sized inline request
instead of using a supplied reply chunk.
Without CCP, however, both sides are guessing the other's inline
threshold. To maintain 100% interoperability, a client endpoint must
always provide a Reply chunk, and a server endpoint must always use
it. However, this requirement can be very inefficient. A middle
ground must be reached.
4.3.1. Recommendations
To provide a stronger guarantee of interoperation while ensuring
efficient operation, RFC 5666bis should explicitly specify when a
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client must offer a Reply chunk, and when a server must use an
offered Reply chunk.
4.4. Computing Credit Values
The third paragraph of Section 3.3 of RFC 5666 leaves open the exact
mechanism of how often the requested and granted credit limits are
supposed to be adjusted. A reader might believe that these values
are adjusted whenever an RPC call or reply is received, to reflect
the number of posted receive buffers on each side.
Although adjustments are allowed by RFC 5666 due to changing
availability of resources on either endpoint, current implementations
use a fixed value. Advertised credit values are always the sum of
the in-process receive buffers and the ready-to-use receive buffers.
4.4.1. Recommendations
RFC 5666bis should clarify the method used to calculate these values.
RFC 5666bis might also discuss how flow control is impacted when a
server endpoint utilizes a shared receive queue.
4.5. Race Windows
The second paragraph of RFC 5666 Section 3.3 says:
Additionally, for protocol correctness, the RPC server must always
be able to reply to client requests, whether or not new buffers
have been posted to accept future receives.
It is true that the RPC server must always be able to reply, and that
therefore the client must provide an adequate number of receive
buffers. The dependent clause "whether or not new buffers have been
posted to accept future receives" is problematic, however.
It's not clear whether this clause refers to a server leaving even a
small window where the sum of posted and in-process receive buffers
is less than the credit limit; or refers to a client leaving a window
where the sum of posted and in-process receive buffers is less than
its advertised credit limit. In either case, such a window could
result in lost messages or be catastrophic for the transport
connection.
4.5.1. Recommendations
Clarify or remove the dependent clause in the section in RFC 5666bis
that is equivalent to RFC 5666 Section 3.3.
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5. Pre-requisites for NFSv4
5.1. Multiple RDMA-eligible Arguments and Results
One NFSv4 COMPOUND may include more than one NFSv4 operation that
conveys RDMA-eligible arguments or replies. There may be additional
considerations when marshaling or decoding such compounds on RPC-
over-RDMA Version One transports.
5.1.1. Recommendations
Additional review of RFC 5666 and prototyping may be needed to
understand if additional protocol requirements are necessary when
multiple read chunks (Read list containing chunks with more than one
Position value) or multiple write chunks (Write list containing
multiple chunk arrays) are present.
More discussion and thought needs to go into handling an NFSv4
COMPOUND reply conveying more than one bulk data result object. When
operation results are defined as XDR unions, it can be ambiguous
which bulk data result object belongs to which Write list entry.
5.2. Bi-directional Operation
NFSv4.1 moves the backchannel onto the same transport as forward
requests [RFC5661]. Typically RPC client endpoints do not expect to
receive RPC call messages. To support NFSv4.1 callback operations,
client and server implementations must be updated to support bi-
directional operation.
Because of RDMA's unique requirements to pre-post receive resources,
special considerations are needed for bi-directional operation.
Conventions have been provided to allow bi-direction, with a limit on
backchannel message size, such that no changes to the RPC-over-RDMA
Version One protocol are needed [I-D.ietf-nfsv4-rpcrdma-bidirection].
5.2.1. Recommendations
RFC 5666bis should reference or include an informational
specification of backwards-direction RPC requests.
5.3. Missing NFS Binding Specifications
To fully support minor versions of NFSv4 on RDMA transports, RFC 5666
requires an Upper Layer Binding Specification for the following
cases. This work is out of scope for RFC 5666bis.
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o NFS ancillary protocols that are not specified in a published IETF
standard, but that are typically conveyed on the same transport as
NFS (e.g. NFSACL)
o NFS ancillary protocols that are not specified in a published IETF
standard, but that can benefit from the low latency operation of
RDMA transports (e.g. NLM)
o NFSv4 minor versions one and newer
o Existing and new pNFS layouts
o NFS protocol extensions that do not increment the minor version
6. Requirements for Upper Layer Binding Specifications
RFC 5666 requires a Binding specification for any RPC program wanting
to use RPC-over-RDMA. The requirement appears in two separate
places: The fourth paragraph of Section 3.4, and the final paragraph
of Section 3.6. As critical as it is to have a Binding
specification, RFC 5666's text regarding these specifications is
sparse and not easy to find.
6.1. Organization Of Binding Specification Requirements
Throughout RPC 5666, various Binding requirements appear, such as:
The mapping of write chunk list entries to procedure arguments
MUST be determined for each protocol.
A similar specific requirement for read list entries is missing.
Usually these statements are followed by a reference to the NFS
Binding specification [RFC5667]. There is no summary of these
requirements, however.
Additional advice appears in the middle of Section 3.4:
It is NOT RECOMMENDED that upper-layer RPC client protocol
specifications omit write chunk lists for eligible replies,
This requirement, being in the middle of a dense paragraph about how
write lists are formed, is easy for an author of Upper Layer Binding
specifications to miss.
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6.1.1. Recommendations
RFC 5666bis should specify explicit generic requirements for what
goes in an Upper Layer Binding specification in one separate section.
In particular, move the third, fourth and fifth paragraph of RFC 5666
Section 3.4 to this new section discussing Binding specification
requirements.
6.2. RDMA Eligibility
The third paragraph of Section 3.4 states that any object MAY be
chosen for chunking (RDMA eligibility) in any given message. That
paragraph also states:
Typically, only those opaque and aggregate data types that may
attain substantial size are considered to be eligible.
Further advice about RDMA eligibility does not appear. However it is
safe to say that object size is not the only consideration for RDMA
eligibility.
For instance, an NFS READDIR result can be large, but typically a
server copies this result piecemeal into place, encoding each
section; and the receiving client must perform the converse actions.
Though there is potentially a large amount of data, the benefit of an
RDMA transfer is lost because of the need for both host CPUs to be
involved in marshaling and decoding.
6.2.1. Recommendations
RFC 5666bis should define what an Upper Layer Binding is, and how it
may be specified.
RFC 5666bis should explicitly specify that an Upper Layer Binding is
required for every RPC program interested in operating on RDMA
transports. Separate bindings may be required for different versions
of that program.
RFC 5666bis should provide generic guidance about what makes a
procedure argument or result eligible for RDMA transfer.
RFC 5666bis should state that the eligibility of any object not
mentioned explicitly in an ULB is "not eligible." The exception is
that Position Zero read chunks and Reply chunks may contain any and
all argument and result objects regardless of their RDMA eligibility.
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RFC 5666bis should remind authors of Upper Layer Bindings that the
Reply chunk and Position Zero read chunks are expressly not for
performance-critical Upper Layer operations.
6.3. Binding Specification Completion Assessment
RFC 5666 Section 3.4 states:
Typically, only those opaque and aggregate data types that may
attain substantial size are considered to be eligible. However,
any object MAY be chosen for chunking in any given message.
Chunk eligibility criteria MUST be determined by each upper-layer
in order to provide for an interoperable specification.
An Upper Layer Binding specification should consider each data type
in the Upper Layer's XDR definition, in particular compound types
such as arrays and lists, when restricting what arguments and results
are eligible for RDMA transfer.
In addition, there are requirements related to using NFS with RPC-
over-RDMA in [RFC5667], and there are some in [RFC5661]. It could be
helpful to have guidance about what kind of requirements belong in an
Upper Layer Binding specification versus what belong in the Upper
Layer Protocol specification.
6.3.1. Recommendations
RFC 5666bis should describe what makes a Binding specification
complete (i.e. read for publication).
7. Removal of Unimplemented Protocol Features
7.1. Read-Read Transfer Model
All existing RPC-over-RDMA Version One implementations use a Read-
Write data transfer model. The server endpoint is responsible for
initiating all RDMA data transfers. The Read-Read transfer model has
been deprecated, but because it appears in RFC 5666, implementations
are still responsible for supporting it. By removing the
specification and discussion of Read-Read, the protocol and
specification can be made simpler and more clear.
7.1.1. Recommendations
Remove Read-Read from RFC 5666bis, in particular from its equivalent
of RFC 5666 Section 3.8. Reserve RDMA_DONE and make it unused.
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7.2. RDMA_MSGP
RDMA_MSGP is typically difficult to implement in requesters, and the
author has found none that do. Responders are required to accept
RDMA_MSGP, though most do not take advantage of it.
Also, notably, without CCP, there is no way for peers to discover a
server endpoint's preferred alignment parameters, unless the
implementation provides an administrative interface for specifying a
remote's alignment parameters. RDMA_MSGP is useless without that a
priori knowledge.
7.2.1. Recommendations
RFC 5666bis should allow implementations that choose not to implement
CCP to not implement RDMA_MSGP. Or, RFC 5666bis should remove
RDMA_MSGP.
8. Optional Additions To The Protocol
These items might be beyond the scope of RFC 5666bis because the
required protocol changes could render existing implementations non-
interoperable, or require a protocol version increment.
8.1. Support For GSS-API With RPC-Over-RDMA
Section 11 of RFC 5666 introduces the concept of employing RPCSEC_GSS
[RFC2203] with an RPC-over-RDMA transport. However, it recommends
using non-GSS-based security mechanisms to retain the efficiency
benefits of RDMA transfer.
In some deployments, the use of GSS-based Kerberos integrity or
privacy is a fixed requirement. One existing RPC-over-RDMA
implementation has chosen to send all integrity and privacy-protected
RPC calls and replies via long messages. The requirement to use long
replies may present an interoperability problem to implementations
that choose not to use long messages in most cases, even for GSS-
wrapped RPC operations.
Another implementation is exploring the possibility of offloading
integrity and privacy computation to the RNIC.
Further discussion and prototyping of RPC-over-RDMA with GSS-API
[RFC2743] is needed. It is desirable to have done this before RFC
5666bis is complete, although it would be a large undertaking.
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8.2. Remote Invalidation
On-the-fly memory registration must be performed when read or write
chunks are transferred as part of an RPC request. A registration
cost is incurred before the RPC call is sent, and an invalidation
cost is incurred after the RPC reply is received.
To relieve the client of the cost of the latter, it is possible for
the server endpoint to ask the client endpoint's RNIC to invalidate
the registered memory associated with an RPC, as part of sending the
RPC reply. When the server performs this invalidation, the client is
no longer required to invalidate during RPC reply processing,
avoiding the cost of that extra operation before retiring the RPC.
We'll refer to the server invalidating a client's R_key as "remote
invalidation."
To perform remote invalidation, the server uses a Send with
Invalidate operation instead of a plain Send operation when conveying
an RPC reply. A Send with Invalidate operation targets a single
R_key that the client's HCA is to invalidate (see [RFC5040]). The
server can invalidate memory regions associated with either read or
write chunks sent by the client.
When an RDMA SEND operation arrives at a client, the client endpoint
receives a completion for a pending receive operation. The
completion indicates whether the server used a plain Send or a Send
With Invalidate. The completion may also indicate which R_key the
server chose to invalidate.
8.2.1. Hardware Support
Not all RNICs support receiving an RDMA SEND that requests an R_key
invalidation. An RPC-over-RDMA client endpoint must somehow indicate
to RPC-over-RDMA servers that Send with Invalidate may be used
instead of Send. Otherwise the client's HCA would reject the Send
with Invalidate conveying the RPC reply, and the RPC (and possibly
the transport connection) would fail.
8.2.2. Avoiding Spurious Invalidation
In some cases, an RPC-over-RDMA client might not wish a server to
perform R_key invalidation, ever. For instance, if the client uses a
global or shared R_key to give the server access to its memory, other
users of the R_key would be adversely affected if the R_key suddenly
became invalid.
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8.2.3. Invalidating Multiple R_keys
Send with Invalidate takes only a single R_key, but an RPC-over-RDMA
client might have registered several memory regions for an RPC, each
with its own R_key.
For example, an RPC request may require a Read list and a Reply
chunk. Read chunks must be registered for read access, but a Reply
chunk is registered to allow writes. Thus two R_keys are required in
this case.
RPC-over-RDMA clients must be prepared to receive RPC replies where
one R_key has been invalidated but others have not.
8.2.4. Invalidation Races
The protocol design must prevent the possibility that the server
might invalidate an R_key that the client has recycled and is still
actively using.
8.2.5. Backward Compatibility
For backward compatibility, RPC-over-RDMA servers must not use Send
with Invalidate when an RPC-over-RDMA client endpoint is not prepared
to sort out which memory regions have been remotely invalidated.
For backward compatibility, RPC-over-RDMA clients must not assume
that an RPC-over-RDMA server endpoint has performed Send with
Invalidate.
8.2.6. Conclusion
It is possible that to address all of the above issues, a change must
be made to the on-the-wire RPC-over-RDMA Version One protocol.
However if no change is required, remote invalidation could be
successfully introduced to the RPC-over-RDMA Version One protocol.
8.3. Work Cancellation
Rarely, an RPC consumer might want to cancel outstanding work. An
application might exit while there are pending RPC operations, for
example, if a software fault or user-generated interrupt occurs. Or,
the RPC service might be slow or unresponsive, and the application
might have placed time-limits that pre-maturely retire RPCs that take
too long to complete.
As a result of canceled work, the client endpoint may tear down
registered RDMA memory regions before the server endpoint has
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performed RDMA operations on the memory. When the server endpoint
attempts to complete the requested work, RDMA operations will
encounter asynchronous memory protection errors because the R_keys
the server has are no longer valid.
This can be fatal for the transport connection. Any other ongoing
work on that connection must be redriven when a new transport
connection has been established, opening a window for RPC requests to
be repeated on the server.
Further, regular and repetitive cancellations of work (for example,
an application repeatedly encountering a segmentation fault) could
have an adverse impact on other work sharing the transport
connection.
One way to avoid this hazard would be to provide a way for requesters
to signal to responders that an RPC reply is no longer required.
This is not a perfect solution, as a work cancellation request can
race with the RPC reply it is trying to cancel.
A work cancellation request might be plumbed in as a new rdma_proc
type. A client would be responsible for keeping receive buffers and
memory regions associated with an RPC until the server has responded
to the cancellation request.
9. Security Considerations
There are no security considerations at this time.
10. IANA Considerations
This document does not require actions by IANA.
11. Acknowledgements
The author gratefully acknowledges the contributions of Dai Ngo,
Karen Deitke, Chunli Zhang, Mahesh Siddheshwar, Dominique Martinet,
and William Simpson.
The author also wishes to thank Dave Noveck and Bill Baker for their
unwavering support of this work. Special thanks go to nfsv4 Working
Group Chair Spencer Shepler and nfsv4 Working Group Secretary Tom
Haynes for their support.
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12. References
12.1. Normative References
[RFC1813] Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
Version 3 Protocol Specification", RFC 1813, DOI 10.17487/
RFC1813, June 1995,
<http://www.rfc-editor.org/info/rfc1813>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2203] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, DOI 10.17487/RFC2203, September
1997, <http://www.rfc-editor.org/info/rfc2203>.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, DOI 10.17487/
RFC2743, January 2000,
<http://www.rfc-editor.org/info/rfc2743>.
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <http://www.rfc-editor.org/info/rfc4506>.
[RFC5040] Recio, R., Metzler, B., Culley, P., Hilland, J., and D.
Garcia, "A Remote Direct Memory Access Protocol
Specification", RFC 5040, DOI 10.17487/RFC5040, October
2007, <http://www.rfc-editor.org/info/rfc5040>.
[RFC5531] Thurlow, R., "RPC: Remote Procedure Call Protocol
Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
May 2009, <http://www.rfc-editor.org/info/rfc5531>.
[RFC5661] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
<http://www.rfc-editor.org/info/rfc5661>.
[RFC5666] Talpey, T. and B. Callaghan, "Remote Direct Memory Access
Transport for Remote Procedure Call", RFC 5666, DOI
10.17487/RFC5666, January 2010,
<http://www.rfc-editor.org/info/rfc5666>.
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[RFC5667] Talpey, T. and B. Callaghan, "Network File System (NFS)
Direct Data Placement", RFC 5667, DOI 10.17487/RFC5667,
January 2010, <http://www.rfc-editor.org/info/rfc5667>.
12.2. Informative References
[I-D.ietf-nfsv4-rpcrdma-bidirection]
Lever, C., "Size-Limited Bi-directional Remote Procedure
Call On Remote Direct Memory Access Transports", draft-
ietf-nfsv4-rpcrdma-bidirection-01 (work in progress),
September 2015.
Author's Address
Charles Lever
Oracle Corporation
1015 Granger Avenue
Ann Arbor, MI 48104
US
Phone: +1 734 274 2396
Email: chuck.lever@oracle.com
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