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Re: [linux-lvm] LVM mirroring questions...



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.

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