Re: [PATCH] xfs: Don't flush inodes when project quota exceeded

[Jan Kara <jack@suse.cz>]

On Tue 20-11-12 10:15:11, Ben Myers wrote:
> Hi Jan,
> 
> On Mon, Nov 19, 2012 at 10:39:13PM +0100, Jan Kara wrote:
> > On Tue 13-11-12 01:36:13, Jan Kara wrote:
> > > When project quota gets exceeded xfs_iomap_write_delay() ends up flushing
> > > inodes because ENOSPC gets returned from xfs_bmapi_delay() instead of EDQUOT.
> > > This makes handling of writes over project quota rather slow as a simple test
> > > program shows:
> > > 	fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
> > > 	for (i = 0; i < 50000; i++)
> > > 		pwrite(fd, buf, 4096, i*4096);
> > > 
> > > Writing 200 MB like this into a directory with 100 MB project quota takes
> > > around 6 minutes while it takes about 2 seconds with this patch applied. This
> > > actually happens in a real world load when nfs pushes data into a directory
> > > which is over project quota.
> > > 
> > > Fix the problem by replacing XFS_QMOPT_ENOSPC flag with XFS_QMOPT_EPDQUOT.
> > > That makes xfs_trans_reserve_quota_bydquots() return new error EPDQUOT when
> > > project quota is exceeded. xfs_bmapi_delay() then uses this flag so that
> > > xfs_iomap_write_delay() can distinguish real ENOSPC (requiring flushing)
> > > from exceeded project quota (not requiring flushing).
> > > 
> > > As a side effect this patch fixes inconsistency where e.g. xfs_create()
> > > returned EDQUOT even when project quota was exceeded.
> >   Ping? Any opinions?
> 
> I think that there may be good reason to flush inodes even in the project quota
> case.  Speculative allocation beyond EOF might need to be cleaned up.  I'm all
> for passing back some data about why we hit ENOSPC.  Then we can combine this
> with Brian Foster's work and flush only inodes that touch a given project,
> user, or group quota.
  Yes, I agree flushing might be useful even for project quota but then why
don't we flush inodes also for user quota? Also the performance impact
is really huge - and here I agree that unless you are writing over NFS you
won't notice because only NFS tries to push X MB to the filesystem page by
page only to get ENOSPC each time... And NFS is arguably doing a stupid
thing but it is a common setup and you don't always have the freedom to fix
clients to be more clever. So I'd be happy if XFS accomodated such use.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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