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Re: XFS mount failuer on RAID5

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: XFS mount failuer on RAID5
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Sun, 18 Oct 2009 23:55:38 -0400
Cc: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20091019013018.GQ9464@xxxxxxxxxxxxxxxx>
References: <389deec70910152009i7656aaf2s9d929b8b68e4e740@xxxxxxxxxxxxxx> <200910161019.10286@xxxxxx> <4AD890D1.4010208@xxxxxxxxxxx> <200910172327.05122@xxxxxx> <20091019013018.GQ9464@xxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.19 (2009-01-05)
On Mon, Oct 19, 2009 at 12:30:18PM +1100, Dave Chinner wrote:
> 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

This should not happen anymore with 2.6 series kernels.

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