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

Szabolcs Szakacsits
2008-08-21T17:06:22Z

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.

> > If so, can you reduce the depth to something less than 4 and see what
> > difference that makes?
>
> Just to point out - this is not a new problem - I can reproduce
> it on 2.6.24 as well as 2.6.26. Likewise, my laptop shows XFS
> being faster than ext3 on both 2.6.24 and 2.6.26. So the difference
> is something related to the disk subsystem on the server....

XFS definitely stalls somewhere: stats show virtually no CPU usage and no
time spent waiting for IO. No file system produces similar output.

procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
0 0 0 3146180 7848 600868 0 0 0 4128 790 549 0 2 98 0
0 0 0 3145200 7848 601524 0 0 0 2372 766 516 0 2 98 0
1 0 0 3144328 7848 602260 0 0 0 2924 792 542 1 2 98 0
0 1 0 3143824 7856 602664 0 0 0 4116 732 426 0 2 53 45
1 0 0 3143068 7856 603136 0 0 0 4676 756 534 0 3 95 1
0 0 0 3142652 7856 603540 0 0 0 6577 756 436 0 0 100 0
0 0 0 3141952 7856 604100 0 0 0 5840 764 498 1 3 96 0
0 0 0 3141424 7856 604544 0 0 0 4752 761 386 0 0 99 0
0 0 0 3140860 7856 604916 0 0 0 6477 785 495 0 1 98 0
0 0 0 3139980 7856 605468 0 0 0 2840 743 370 1 2 97 0
0 0 0 3138464 7856 606884 0 0 0 4902 795 421 0 4 96 0
0 0 0 3137636 7856 607696 0 0 0 4364 739 395 0 1 99 0
0 0 0 3136520 7856 608220 0 0 0 6160 774 566 0 2 97 0

Szaka

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