Re: NFS Sync with External Journal

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Jun 06, 2011 at 01:12:51PM -0700, Wendy Cheng wrote:
> You'll probably get better answer(s) from ext3 user mailing list
> .... it is more about how journaling works for the specific file
> system.

Yes, though leave linux-nfs on the cc: as I'd be interested what you
find out.

> In ext3 case, I believe "sync" forces data getting flushed to the file
> system *regardless* which journal mode is chosen. Using an external
> journal device, particularly on SSD,  does help but the performance
> gain is limited by the amount of data that needs to be written into
> the file system itself.

> > /etc/exports:
> > /plain          *(rw,async,no_subtree_check,no_root_squash)
> > /split          *(rw,async,no_subtree_check,no_root_squash) # (FS with external Journal)
> >
> > Client mounts were done simply with -o 'rw,rsize=32768,wsize=32768'
> >
> > Benchmark results:
> > Plain Ext3, data=ordered export=sync, write speed 56-62MB/sec
> > Split Ext3, data=journal export=sync, write speed = 46-50MB/sec
> >
> > For reference:
> > Plain Ext3, data=ordered export=async, write speed 111MB/sec
> > Split Ext3, data=journal export=async, write speed 110MB/sec

What exactly is your test?

For sufficiently large sequential writes, I wouldn't actually have
expected sync vs. async to make much difference: eventually you're
limited by the drive speed (I'm assuming your drive does ~60MB/s write
througput?).  And individual writes (for NFS v3 and higher) aren't
necessarily required to be synchronous.

A better test would be creating or destroying a bunch of small files, as
create and unlink are synchronous (the nfs server won't return, in the
sync case, before each create and unlink actually hits the disk).

--b.