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

On Thu, Aug 21, 2008 at 08:33:50PM +0300, Szabolcs Szakacsits wrote:

On Thu, 21 Aug 2008, Szabolcs Szakacsits wrote:

quoted

On Thu, 21 Aug 2008, Dave Chinner wrote:

quoted

On Thu, Aug 21, 2008 at 04:04:18PM +1000, Dave Chinner wrote:

quoted

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