Issues Related to RPC-over-RDMA Internode Round Trips
draft-dnoveck-nfsv4-rpcrdma-rtissues-03

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Network File System Version 4                                  D. Noveck
Internet-Draft                                         February 27, 2017
Intended status: Informational
Expires: August 31, 2017

         Issues Related to RPC-over-RDMA Internode Round Trips
                draft-dnoveck-nfsv4-rpcrdma-rtissues-03

Abstract

   As currently designed and implemented, the RPC-over-RDMA protocol
   requires use of multiple internode round trips to process some common
   operations.  For example, NFS WRITE operations require use of three
   internode round trips.  This document looks at this issue and
   discusses what can and what should be done to address it, both within
   the context of an extensible version of RPC-over-RDMA and potentially
   outside that framework.

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Noveck                   Expires August 31, 2017                [Page 1]
Internet-Draft         RPC/RDMA Round-trip Issues          February 2017

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Table of Contents

   1.  Preliminaries . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
     1.2.  Introduction  . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Review of the Current Situation . . . . . . . . . . . . . . .   3
     2.1.  Troublesome Requests  . . . . . . . . . . . . . . . . . .   3
     2.2.  WRITE Request Processing Details  . . . . . . . . . . . .   4
     2.3.  READ Request Processing Details . . . . . . . . . . . . .   5
   3.  Near-term Work  . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Target Performance  . . . . . . . . . . . . . . . . . . .   7
     3.2.  Message Continuation  . . . . . . . . . . . . . . . . . .   8
     3.3.  Send-based DDP  . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Feature Synergy . . . . . . . . . . . . . . . . . . . . .   9
     3.5.  Feature Selection and Negotiation . . . . . . . . . . . .  10
   4.  Possible Future Development . . . . . . . . . . . . . . . . .  12
   5.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Preliminaries

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 [RFC2119].

1.2.  Introduction

   When many common operations are performed using RPC-over-RDMA,
   additional inter-node round-trip latencies are required to take
   advantage of the performance benefits provided by RDMA Functionality.

   While the latencies involved are generally small, they are a reason
   for concern for two reasons.

   o  With the ongoing improvement of persistent memory technologies,
      such internode latencies, being fixed, can be expected to consume
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