NFS server stalls with 2.4.18 + XFS 1.1

[James Pearson <james-p@xxxxxxxxxxxxxxxxxx>]

We have noticed a problem with a couple of our NFS servers (running
RedHat 7.2 with a stock 2.4.18 kernel with XFS v1.1) whereby NFS access
slows to a crawl or stalls.

The exported filesystem(s) are XFS with 8 nfsd's running - when we have
the problem the load average is about 8 - but CPU usage, disk access and
network traffic are minimal.

I found, by accident, that running the command 'sync' appears to 'fix'
the situation...

I'm not sure if this is an XFS or NFS related problem (hence posting to
both lists).

Thanks

James Pearson