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Date: Fri, 11 Apr 2008 18:18:20 +0200
From: Luca Berra <bluca@comedia.it>
Subject: Re: [linux-lvm] LVM mirroring questions...
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To: linux-lvm@redhat.com

On Tue, Apr 08, 2008 at 07:25:05AM -0400, Stuart D. Gathman wrote:
>On Tue, 8 Apr 2008, Jess Bromley wrote:
>
>> Also does the fact that software RAID doesn't use a disk log mean that
>> LVM is actually better/faster than RAID at correctly recovering from
>> crashes that leave the mirrored disks out of sync?
>
>The md driver doesn't track which blocks are out of sync.  It recovers
>from a crash by copying the entire partition.  On recent versions, it
>tracks how far it has gotten on the resync in the md superblock, so that it
>doesn't have to start from scratch on a reboot.

this is not entirely correct.
md has a bitmap feature that is used to track which blocks are out of
sync.

>The md driver keeps its superblock at the *end* of the partition.  This
>means that you can mount any filesystem/pv as the raw partition.
again, newer version of md allow to put the superblock at the beginning
of the device (or 4k from the beginning)

>This requires that LVM give md devices preference when scanning for
>PVs to work correctly :-)

this should be the default in lvm.

L.

-- 
Luca Berra -- bluca@comedia.it
        Communication Media & Services S.r.l.
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