Re: Read/Write NFS I/O performance degraded by FLUSH_STABLE page flushing

Trond Myklebust
2009-05-30T12:43:41Z

On May 30, 2009, at 8:26, Trond Myklebust <trond.myklebust@fys.uio.no>
wrote:
>
> Firstly, the server only uses O_SYNC if you turn off write gathering
> (a.k.a. the 'wdelay' option). The default behaviour for the Linux nfs
> server is to always try write gathering and hence no O_SYNC.
>
> Secondly, even if it were the case, then this does not justify
> changing
> the client behaviour. The NFS protocol does not mandate, or even
> recommend that the server use O_SYNC. All it says is that a stable
> write
> and an unstable write+commit should both have the same result: namely
> that the data+metadata must have been flushed to stable storage. The
> protocol spec leaves it as an exercise to the server implementer to do
> this as efficiently as possible.
>

Speaking of write gathering... Are we sure that heuristic that checks
i_writecount isn't introducing spurious 10ms delays here? It seems odd
for the server to do write gathering on nfsv3 writes: if the client
wants to send more writes, it will set the unstable flag...

Trond