linux-kernel - Re: [PATCH 00/16] DRBD: a block device for HA clusters

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Date:	Wed, 6 May 2009 10:17:56 +0200
From:	Philipp Reisner <philipp.reisner@...bit.com>
To:	James Bottomley <James.Bottomley@...senpartnership.com>
Cc:	david@...g.hm, Willy Tarreau <w@....eu>,
	Bart Van Assche <bart.vanassche@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
	Greg KH <gregkh@...e.de>, Neil Brown <neilb@...e.de>,
	Sam Ravnborg <sam@...nborg.org>, Dave Jones <davej@...hat.com>,
	Nikanth Karthikesan <knikanth@...e.de>,
	"Lars Marowsky-Bree" <lmb@...e.de>,
	Kyle Moffett <kyle@...fetthome.net>,
	Lars Ellenberg <lars.ellenberg@...bit.com>
Subject: Re: [PATCH 00/16] DRBD: a block device for HA clusters

[...]
>
> Well, you have to agree that during a resync from the activity log,
> which plays up the primary disk from one end to another, the secondary
> is completely corrupt if a primary failure occurs before the resync
> completes.  That's something that's triggered by a network outage, and
> so is a far more common event than cascading dual failures.  It's all
> really a question of where you focus your effort to eliminate the corner
> cases.
>

I fully agree. Just not not leave this unanswered: With DRBD we provide
a snapshot-resync-target handler. By using LVM's snapshotting
mechanism a snapshot is taken before it becomes resync-target. In
case the resync completes gracefully, the snapshot is automatically
removed.

Which is still inferior to a full transaction log on the secondary.

-Phil

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