[Beowulf] Help for terrible NFS write performance

Skylar Thompson skylar at cs.earlham.edu
Fri Aug 21 11:03:00 PDT 2009

Orion Poplawski wrote:
> Okay, how afraid should I be of using async?  man exports states:
>        async  This option allows the NFS server to violate the NFS
> protocol
>               and  reply to requests before any changes made by that
> request
>               have been committed to stable storage (e.g. disc drive).
>               Using this option might improve  performance  with
> version  2
>               only,  but  at the cost that an unclean server restart
> (i.e. a
>               crash) can cause data to be lost or corrupted.
> I tend to like to avoid data loss or corruption.
> Thanks!

It depends on your applications. async doesn't remove the capability of
writing synchronously to an NFS export, but it removes the implicit sync
after every write. If your applications know to fsync() when they
absolutely need to have stuff written to disk (and of course check errno
after fsync()), the server will respect that and flush the data for that
file and only reply if the data was flushed properly.

NB: This likely depends on both your NFS server and client
implementations. Use with caution. YMMV.

-- Skylar Thompson (skylar at cs.earlham.edu)
-- http://www.cs.earlham.edu/~skylar/

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