Internet Engineering Task Force M. Eisler, Ed.
Internet-Draft NetApp
Intended status: Informational October 19, 2009
Expires: April 22, 2010
Requirements for NFSv4.2
draft-eisler-nfsv4-minorversion-2-requirements-01
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Abstract
This document proposes requirements for NFSv4.2.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. Efficiency and Utilization Requirements . . . . . . . . . . . . 3
2.1. Capacity . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Network Bandwidth and Processing . . . . . . . . . . . . . 5
3. Flash Memory Requirements . . . . . . . . . . . . . . . . . . . 5
4. Compliance . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Incremental Improvements . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
NFSv4.1 [I-D.ietf-nfsv4-minorversion1] is an approved specification.
The NFSv4 [RFC3530] community has indicated a desire to continue
innovating NFS, and specifically via a new minor version of NFSv4,
namely NFSv4.2. The desire for future innovation is primarily driven
by two trends in the storage industry:
o High efficiency and utilization of resources such as, capacity,
network bandwidth, and processors.
o Solid state flash storage which promises faster throughput and
lower latency than magnetic disk drives and lower cost than
dynamic random access memory.
Secondarily, innovation is being driver by the trend to stronger
compliance with information management. In addition, as might be
expected with a complex protocol like NFSv4.1, implementation
experience has shown that minor changes to the protocol would be
useful to improve the end user experience.
This document proposes requirements along these four themes, and
attempts to strike a balance between stating the problem and
proposing solutions. With respect to the latter, some thinking among
the NFS community has taken place, and a future revision of this
document will reference embodiments of such thinking.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Efficiency and Utilization Requirements
2.1. Capacity
Despite the capacity of magnetic disk continuing to increase at
exponential rates, the storage industry is under pressure to make the
storage of data increasingly efficient, so that more data can be
stored. The driver for this counter-intuitive demand is that disk
access times are not improving anywhere near as quickly as
capacities. The industry has responded to this development by
increasing data density via limiting the number of times a unique
pattern of data is stored in a storage device. For example some
storage devices support de-duplication. When storing two files, a
storage device might compare them for shared patterns of data, and
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store the pattern just once, and setting reference counts on the
blocks of the unique pattern to two. With de-duplication the number
of times a storage device has to read a particular pattern would be
reduced to just once, thus improving average access time.
For a file access protocol such as NFS, there are several implied
requirements for addressing this capacity efficiency trend:
o The "space_used" attribute of NFSv4 does not report meaningful
information. Removing a file with a "space_used" value of X bytes
does not mean that the file system will see an increase of X
available bytes. Providing more meaningful information is a
requirement.
o Because it is probable, especially for applications such as
hypervisors, the NFSv4 client is accessing multiple files with
shared blocks of data, it is in the interest of the client and
server for the client to know which blocks are share so that they
are are not read multiple times, and not cached multiple times.
Providing a block map of shared blocks is a requirement.
o If an NFSv4 client is aware of which patterns exist on which
files, when it wants to write pattern X to file B to offset J, and
it knows that X also exists in offset I of file A, then if it can
advise the server of its intent, the server can arrange for
pattern X to appear in file A being a zero copy. Even if the
server does not support de-duplication, it can at least perform a
local copy that saves network bandwidth and processor overhead on
the client and server.
o File holes are patterns of zeros that in some file systems do are
unallocated blocks. In a sense, holes are the ultimate de-
duplicated pattern. While proposals to extend NFS to support hole
punching have been around since the 1980s, until recently there
have not been NFS clients that could make use of hole punching.
The Information Technology (IT) trend toward virtualizing
operating environments via hypervisors has resulted in a need for
hypervisors to translate a (virtual) disk command to free a block
into an NFS request to free that block. On the read side, if a
file contains holes, then again, as the ultimate in de-
duplication, it would be better for the client to be told the
region it wants to read has a hole, instead of of returning long
arrays of zero bytes. Even if a server does not support holes on
write or read, avoiding the transmission of zeroes will save
network bandwidth and reduce processor overhead.
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2.2. Network Bandwidth and Processing
The computational capabilities of processors continues to grow at an
exponential rate. However, as noted previously, because disk access
times are not showing a commensurate exponential decrease, disk
performance is not tracking processor performance. In addition,
while network bandwidth is exponentially increasing, unlike disk
capacities and processor bandwidth, the improvement is not seen on a
1-2 year cycle, but happens on something closer to a 10 year cycle.
The lag between disk and network performance compared to processor
performance means that there is often a discontinuity between the
processing capabilities of NFS clients and the speed at which they
can extract data from an NFS server. For some use cases, much of the
data that is read by one client from an NFS server also needs to be
read by other clients. Re-reading this data is will result in a
waste of the network bandwidth and processing of the NFS server.
This same observation has driven the creation of peer-to-peer content
distribution protocols, where data is directly read from peers rather
than servers. It is apparent that a similar technique could be used
to offload primary storage.
The pNFS protocol distributes the I/O to a set of files across a
cluster of data servers. Arguably, its primary value is in balancing
load across storage devices, especially when it can leverage a back
end file system or storage cluster with automatic load balancing
capabilities. In NFSv4.1, no consideration was given to metadata.
Metadata is critical to several workloads, to the point that, as
defined in NFSv4.1, pNFS will not not offer much value in those
cases. The load balancing capabilities of pNFS need to be brought to
metadata.
From an end user perspective, the operations performed on a file
include creating, reading, writing, deleting, and copying. NFSv4 has
operations for all but the last. While file copy has been proposed
for NFS in the past, it was always rejected because of the lack of
Application Programming Interfaces (APIs) within existing operating
environments to send a copy operation. The IT trend toward
virtualization via hypervisors has changed the situation, where the
emerging use case is to copy a virtual disk. The use of a copy
operation will save network bandwidth on the client and server, and
where the server supports it, intra-server file copy has the
potential to avoid all physical data copy.
3. Flash Memory Requirements
Flash memory is rapidly filling the wide gap between expensive but
fast Dynamic Random Access Memory (DRAM) and inexpensive but cheap
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magnetic disk. The cost per bit of flash is between DRAM and disk.
The access time pet bit of flash is between DRAM and disk. This has
resulted in the File access Operations Per Second (FOPS) per unit of
cost of flash exceeding DRAM and disk. Flash can be easily added as
another storage medium to NFS servers, and this does not require a
change to the NFS protocol. However, the value of flash's superior
FOPS is best realized when flash is closest to the application, i.e.
on the NFS client. One approach would be to forgo the use of network
storage and de-evolve back to Direct Attached Storage (DAS).
However, this would require that data protection value that exists in
modern storage devices be brought into DAS, and this is not always
convenient or cost effective. A less traumatic way to leverage the
full FOPS of flash would be for NFSv4 clients to leverage flash for
caching of data.
Today NFSv4 supports whole file delegations for enabling caching.
Such a granularity is useful for applications like user home
directories where there is little file sharing. However, NFS is used
for many more workloads, which include file sharing. In these
workloads, files are shared, whereas individual blocks might not be.
This drives a requirement for sub-file caching.
4. Compliance
New regulations for the IT industry limit who can view what data.
NFSv4 has Access Control Lists (ACLs), but the ACL can be changed by
the nominal file owner. In practice, the end user that owns the file
(essentially, has the right to delete the file or give permissions to
other users), is not the legal owner of the file. The legal owner of
the file wants to control not just who can access the file, but who
they can pass the content of the file to. The IT industry has
addressed this need in the past with notion of security labeling.
Labels are attached to devices, files, users, applications, network
connections, etc. When the labels of two objects match, data can be
transferred from one to another. For example a label called "Secret"
on a file results in only users with a "Secret" security clearance
being allowed to view the file, despite what the ACL says.
To attach a label on a file requires that it be created atomically
with the file, which means that a new RECOMMENDED attribute for a
security label is needed.
5. Incremental Improvements
Implementation experience with NFSv4.1 and related protocols, such as
SMB2, has shown a number of areas where the protocol can be improved.
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o Hints for the type of file access, such as sequential read. While
traditionally NFS servers have been able to detect read-a-head
patterns, with the introduction of pNFS, this will be harder.
Since NFS clients can detect patterns of access, they can advise
servers. In addition, the UNIX/Linux madvise() API is an example
of where applications can provide direct advice to the NFS server.
o Head of line blocking. Consider a client that wants to send a
three operations: a file creation, a read for one megabyte, and a
write for one megabyte. Each of these might be sent on a separate
slot. The client determines that it is not desirable for the read
operation to wait for the write operation to be sent, so it sends
the create. However, it does not want to serialize the read and
write behind the create, so the read gets sent, followed by the
write. On the reply side, the server does not know that client
wants the create satisfied first, so read and write operations are
first processed. By the time the create is performed on the
server, the response to the read is still filling the reply side.
While NFSv4.1 could solve this problem by associating two
connections with the session, and using one connection for create,
and the other for read or write, multiple connections come at a
cost. The requirement is to solve this head of line blocking
problem. Tagging a request as one that should go to the head of
the line for request and response processing is one possible way
to address it.
o pNFS connectivity/access indication. If a pNFS client is given a
layout that directs it to a storage device it cannot access due to
connectivity of access control issues, it has no way in NFSv4.1 to
indicate the problem to the metadata server.
o RPCSEC_GSS sequence window size on backchannel. The NFSv4.1
specification does not have a way to for the client to tell the
server what window size to use on the backchannel. The
specification says that the window size will be the same as what
the server uses. Potentially, a server could use a very large
window size that the client does not want.
o Trunking discovery. The NFSv4.1 specification is long on how a
client verifies if trunking is available between two connections,
but short on how a client can discover destination addresses that
can be trunked. It would be useful if there was a method (such as
an operation) to get a list of destinations that can be session or
client ID trunked, as well as a notification when the set of
destinations changes.
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6. IANA Considerations
None.
7. Security Considerations
None.
8. Acknowledgements
Thanks to Dave Noveck for reviewing this document and providing
valuable feedback.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[I-D.ietf-nfsv4-minorversion1]
Shepler, S., Eisler, M., and D. Noveck, "NFS Version 4
Minor Version 1", draft-ietf-nfsv4-minorversion1-29 (work
in progress), December 2008.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File System
(NFS) version 4 Protocol", RFC 3530, April 2003.
Author's Address
Michael Eisler (editor)
NetApp
5765 Chase Point Circle
Colorado Springs, CO 80919
US
Phone: +1 719 599 8759
Email: mike@eisler.com
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