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Date: Mon, 28 Mar 2011 14:34:39 -0400
From: Mike Snitzer <snitzer@redhat.com>
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Subject: Re: [linux-lvm] Powerfailure and snapshot consistency
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To: LVM general discussion and development <linux-lvm@redhat.com>

On Mon, Mar 28 2011 at  1:24pm -0400,
Stuart D. Gathman <stuart@bmsi.com> wrote:

> On Mon, 28 Mar 2011, Phillip Susi wrote:
> 
> >On 3/27/2011 5:55 PM, Stuart D. Gathman wrote:
> >>Don't need help, just trying to understand the event.  So from what you
> >>said, the problem probably stems from the RAID1 resync, which could
> >>make one PV "older" than the other.
> >
> >The data is written to the primary mirror first, so this can't happen.
> >Proper use of barriers will prevent this whole scenario, so my guess is
> >that you are using ext3 ( which defaults to nobarrier ) or explicitly
> >mounting ext4 with nobarrier.
> 
> Ah, thank you!  Yes, I am using ext3 with EL5.5 defaults, and will now learn
> about the barrier option.  Seems like a good thing to turn on when "replace
> battery" comes up on the ups.

RHEL5.x's DM doesn't support barriers (nor does MD afaik).