How to monitor Linux NFS client load?

[Andrey Borzenkov <arvidjaar@mail.ru>]

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Recently we have the case of very high latencies on NFS reads as 
reported by application (SAP R/3). NFS server was NetApp FAS; according 
to NetApp statistic, average volume read latencies were in order 10ms, 
while SAP stats gave 30-50ms. Systems were interconnected by dedicated 
1Gb/s Cisco switches (3750G) with ca. 30% max load on interfaces.

On advice of my colleague we changed sunrpc.tcp_slot_table_entries from 
default 16 to 128 which seemed to make situation much better - without 
changing load pattern of filer in any visible way.

Now, I can understand, why we observed much higher latency on system and 
why changing (what effectively is) queue depth helped. But I am totally 
frustrated that there does not appear to be *any* possibility to detect 
this situation on Linux side and to get a real numbers of real NFS IO 
latencies or number of requests waiting to be executed (and I do not 
even dream about per-mount point stats).

I am grateful for any hints how can we monitor Linux NFS client and get 
real-life numbers of what happens inside. Thank you!

-andrey

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