From: Dave Chinner <david@fromorbit.com>
To: Michael Monnerie <michael.monnerie@is.it-management.at>
Cc: xfs@oss.sgi.com
Subject: Re: XFS mount failuer on RAID5
Date: Mon, 19 Oct 2009 12:30:18 +1100 [thread overview]
Message-ID: <20091019013018.GQ9464@discord.disaster> (raw)
In-Reply-To: <200910172327.05122@zmi.at>
On Sat, Oct 17, 2009 at 11:27:04PM +0200, Michael Monnerie wrote:
> On Freitag 16 Oktober 2009 Eric Sandeen wrote:
> > Actually -ssize=4k is just fine even on 512 sector disks.
>
> Oh funny. So what's the meaning of this argument then? Or why would one
> set it to 4k? What's the diff with 512b?
A hardware sector is the atomic unit of IO. 4k sectors on 512b
hardware sectors means that a single 4k filesystem sector write is
not necessarily atomic. This can lead to problems with torn writes
at power loss or sub-filesystem-sector data loss/corruption when a
hardware sector goes bad. In general, these are detected no
differently to the same sector loss on a 512b filesystem sector
filesystem.
IIRC, the main reason for 4k sectors on MD RAID5/6 is that changing
the IO alignment from 4k to 512 byte IOs (i.e. sub-page sized)
causes MD to flush and invalidate the stripe cache. Hence every
time XFS writes a super block, AGF, AGFL or AGI, things go much
slower because of this flush/invalidate. By setting the sector size
to 4k, the SB/AGF/AGFL/AGI are all 4k in size and hence IO alignment
never changes and hence performance remains good.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
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next prev parent reply other threads:[~2009-10-19 1:28 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-10-16 3:09 XFS mount failuer on RAID5 hank peng
2009-10-16 8:19 ` Michael Monnerie
2009-10-16 15:27 ` Eric Sandeen
2009-10-17 21:27 ` Michael Monnerie
2009-10-19 1:30 ` Dave Chinner [this message]
2009-10-19 3:55 ` Christoph Hellwig
2009-10-16 8:30 ` Justin Piszcz
2009-10-16 15:28 ` Eric Sandeen
2009-10-16 15:55 ` Eric Sandeen
2009-10-19 0:54 ` hank peng
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