[Lustre-discuss] NFS vs Lustre

[Daniel Kobras]

Hi!

On Sun, Aug 30, 2009 at 04:12:11PM -0500, Nicolas Williams wrote:
> NFSv4 can't handle O_APPEND, and has those close-to-open semantics.
> Those are the two large departures from POSIX in NFSv4.

Along these lines, it's probably worth mentioning commit-on-close as well, an
area where NFS (v3 and v4, optionally relaxed when using write delegations) is
more strict than Posix. This is to make sure that NFS still has the possibility
to notify the user about errors when trying to save their data. Lustre's
standard config follows Posix and allows dirty client-side caches after
close(). Performance improves as a result, of course, but in case something
goes wrong on the net or the server, users potentially lose data just like on
any local Posix filesystem. The difference being that users tend to notice when
their local machine crashes. It's much easier to miss a remote server or a
switch going down, and hence suffer from silent data loss. (Admins will
typically notice, eg. via eviction messages in the logs, but have a hard time
telling whicht files had been affected.) The solution is to fsync() all
valuable data on a Posix filesystem, but that's not necessarily within reach
for an average end user.

Regards,

Daniel.