suddenly slow writes on XFS Filesystem

[Dave Chinner]

On Sun, May 06, 2012 at 09:25:10PM +0200, Stefan Priebe wrote:
> Am 06.05.2012 17:45, schrieb Stan Hoeppner:
> >On 5/6/2012 5:33 AM, Martin Steigerwald wrote:
> >>Am Sonntag, 6. Mai 2012 schrieb Stefan Priebe:
> >>>It is a raid 10 of 20 SATA Disks and i can only write to them with
> >>>about  700kb/s while doing random i/o. I tried vanilla Kernel 3.0.30
> >>>and 3.3.4 - no difference. Writing to another partition on another xfs
> >>>array works fine.
> >>
> >>Additionally what RAID is this? SoftRAID or some - which one? - hardware
> >>RAID controller? And what disks are used, whats the rpm of these?
> >
> >I doubt much of this stuff matters.  Stefan's filesystem is 96% full,
> >w/~200GB free.  This free space is likely heavily fragmented.  If he's
> >doing allocation in this fragmented free space I'd think that would
> >fully explain his write performance dropping off a cliff due to massive
> >head seeking.
> >
> Thanks Stan that's it. After deleting 200GB-300GB it's running fine again.
> 
> What is the general recommandation of free space?

Depends on the size of the filesystem. If you've got a 500TB
filesystem, then running at 98% full (10TB of free space) is not
going to be a big deal. But running at 98% full on a 5TB volume is a
big deal because there is relatively little freespace per AG and it
will get rapidly fragmented.

So for a 5TB volume, I' say don't run sustained operations at over
90% full. Going above this temporarily won't be a problem, but
staying at >95% full will definitely cause accelerated aging of the
filesystem.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com