Re: XFS vs Elevators (was Re: [PATCH RFC] nilfs2: continuous snapshotting file system)

[Dave Chinner <david@fromorbit.com>]

On Thu, Aug 21, 2008 at 08:33:50PM +0300, Szabolcs Szakacsits wrote:
> 
> On Thu, 21 Aug 2008, Szabolcs Szakacsits wrote:
> > On Thu, 21 Aug 2008, Dave Chinner wrote:
> > > On Thu, Aug 21, 2008 at 04:04:18PM +1000, Dave Chinner wrote:
> > > > 
> > > > One thing I just found out - my old *laptop* is 4-5x faster than the
> > > > 10krpm scsi disk behind an old cciss raid controller.  I'm wondering
> > > > if the long delays in dispatch is caused by an interaction with CTQ
> > > > but I can't change it on the cciss raid controllers. Are you using
> > > > ctq/ncq on your machine?  
> > 
> > It's a laptop and has NCQ. It makes no difference if NCQ is enabled or 
> > disabled. The problem seems to be XFS only.
> 
> The 'nobarrier' mount option made a big improvement:
> 
>                     MB/s    Runtime (s)
>                    -----    -----------
>   btrfs unstable   17.09        572
>   ext3             13.24        877
>   btrfs 0.16       12.33        793
>   nilfs2 2nd+ runs 11.29        674
>   ntfs-3g           8.55        865
>   reiserfs          8.38        966
>   xfs nobarrier     7.89        949
>   nilfs2 1st run    4.95       3800
>   xfs               1.88       3901

INteresting. Barriers make only a little difference on my laptop;
10-20% slower. But yes, barriers will have this effect on XFS.

If you've got NCQ, then you'd do better to turn off write caching
on the drive, turn off barriers and use NCQ to give you back the
performance that the write cache used to. That is, of course,
assuming the NCQ implementation doesn't suck....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com