nfs and dropped udp datagrams

[Rick Macklem]

> I wonder how big kern.ipc.maxsockbuf shall be tuned on a busy
> NFS-Server. In the last time, I often see clients loosing connection
> to
> their mounted directories. Furthermore, I see increasing counts for
> upd datagrams "dropped due to full socket buffers" in netstat -s -p
> udp.
> 
> The mbuf situation does not seem to be the reason for the lost
> connections, vmstat -z shows 0 failures in the mbuf section.
> 
> Are there any other tunables which could prevent loss of connection to
> the server? What is a reasonable value for maxsockbuf?
> 
Prior to r213756 the kernel rpc didn't check for the return value from
soreserve() so, if maxsockbuf wasn't large enough or the rsize/wsize was
greater than 8K, it failed and the krpc didn't notice. However, if it
fails, then sesend() drops messages and that causes grief.

I'd just double it on all systems (clients and server), then double it
again if you still see problems.

rick