NFS access slow

[Keith Edmunds <kae@midnighthax.com>]

Accessing disks locally on a server gives read speeds around 100MB/s,
write speeds around 267MB/s.

Mounting the same disks on the same server via NFS (ie, not using the
network at all) gives read speeds around 30MB/s, write speeds around
80MB/s.

That's about 30% of the local access speed.

Is that to be expected? I'd expect a 10-15% slowdown, but not this much.

This is using NFSv4; using NFSv3 improves the speeds slightly (36MB/s
read, 95MB/s write).

Other parameters we've changed, none of which have a significant impact:

 - UDP/TCP
 - rsize and wsize
 - noatime
 - noacl
 - nocto

If that's an unexpected slow down, where should we be looking?

Thanks.