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

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Date:	Mon, 4 May 2009 18:12:52 +0200
From:	Lars Marowsky-Bree <lmb@...e.de>
To:	Lars Ellenberg <lars.ellenberg@...bit.com>,
	Neil Brown <neilb@...e.de>
Cc:	Philipp Reisner <philipp.reisner@...bit.com>,
	linux-kernel@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
	Greg KH <gregkh@...e.de>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Sam Ravnborg <sam@...nborg.org>, Dave Jones <davej@...hat.com>,
	Nikanth Karthikesan <knikanth@...e.de>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	Kyle Moffett <kyle@...fetthome.net>,
	Bart Van Assche <bart.vanassche@...il.com>
Subject: Re: [PATCH 00/16] DRBD: a block device for HA clusters

On 2009-05-03T23:32:31, Lars Ellenberg <lars.ellenberg@...bit.com> wrote:

> Which it could not be while replication link is down,
> so once replication link is back (or remote node is back,
> which is not easily distinguishable just there, blablabla),
> you'd need to fetch the remote bitmap, and merge it with the local
> bitmap (feeding it into bitmap_set_bits),
> then re-attach the "failed" mirror.

Note that this sacrifices transactional consistency on the sync target;
an understandable trade-off (versus recording the stream of writes
entirely, which consumes space and possibly more resync bandwidth), but
a noteworthy one.

> But DRBD as of now does the connection handshake and bitmap exchange in
> kernel.  We wanted to have a fast compression scheme suitable for
> bitmaps, without cpu or memory overhead.  This does it quite nicely.

Sharing the connection between meta- and regular data also avoids some
ordering issues between channels, which probably helps simplify some
aspects of drbd.

Conceivably, the kernel could escalate such metadata/out-of-band
communications to user-space for handling, and user-space would then
afterwards instruct the continuation of the stream processing.

> or the link has been down,
> and the remote side decided to go active with it.

That is arguably a horrible failure on behalf of the cluster stack being
used, but indeed something drbd must be able to recover from.

Regards,
    Lars

-- 
SuSE Labs, OPS Engineering, Novell, Inc.
SUSE LINUX Products GmbH, GF: Markus Rex, HRB 16746 (AG Nürnberg)
"Experience is the name everyone gives to their mistakes." -- Oscar Wilde

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