I'm curious to know if anyone has run across a long-standing problem
we've seen with NFSv3 UDP clients.
Back under RHEL4 (2.6.9-based), NFSv3 UDP mounts had very good
performance with our internal testing. However, any release I've
tested since then (RHEL5, 2.6.31, RHEL6, 3.3, and 3.6) the results
have been poor and very chaotic with wild swings in results,
generally showing around a 25%-30% drop in our internal tests when
compared to TCP.
Our performance test suite typically runs 50 processes doing a
random, mixed client load of read, multi-read, append, and write
operations to an older Netapp filer sprayed across 23 NFSv3 mounted
file systems. We often run with the {r,w}size to 32k (I know, not
usually recommended for UDP, but usually works well for our network)
and up the tunable "sunrpc.udp_slot_table_entries" from 16 to either
64 or 128.
In looking at the statistics, one problem that stands out is the number of
RPC retransmits. On RHEL4 (and also when using a FreeBSD client),
the number of RPC retransmits during our testing is around 500/hr.
With all later Linux kernels, that rate shoots up to 7000-12000/hr.
That still doesn't seem to be much given the number of packets
slung, but I think that points towards where the problem might be,
in the sunrpc network error detection, recovery, and backoff code
(which is completely avoided with TCP mounts).
Since for our work, for other reasons, we've switched over to
using NFSv3 TCP mounts, so I can't justify spending a lot of time
debugging this UDP/RPC problem. However, for example if someone
wants me to try something out and gather some new test results or
a patch to test, I can squeeze that in.
Does anyone know if this problem has a history or has already been
looked at?
Quentin
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