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List:       linux-lvm
Subject:    Re: [linux-lvm] LVM mirror questions
From:       Petr Rockai <prockai () redhat ! com>
Date:       2010-10-07 12:14:43
Message-ID: 87fwwijj2k.fsf () twilight ! int ! mornfall ! net ! 
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Phillip Susi <psusi@cfl.rr.com> writes:
>> LVM has a generation counter as well. It's only updated on metadata
>> writes though, so it doesn't cost anything. (Log is not part of metadata
>> in this sense.) When you lose a leg (and use dmeventd), the metadata on
>> the remaining PVs is updated to say that. The leg is also yanked from
>> the mirror. You can add it as a fresh image (with full resync) if it
>> ever comes back.
>
> Why a full resync?  With mdadm, it just keeps flagging the dirty chunks
> so it only has to copy those when the other disk returns.

Because it's really hard to guarantee anything when you experience a
failure. The safe route is to discard any data on the failed
drive. Certainly better than winding up with a subtly corrupt mirror.

As I said before, if you experience that a lot, you should be using
replication not mirroring.

Yours,
   Petr.

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