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

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Date:	Sun, 03 May 2009 09:21:14 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	david@...g.hm
Cc:	Willy Tarreau <w@....eu>,
	Bart Van Assche <bart.vanassche@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Philipp Reisner <philipp.reisner@...bit.com>,
	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

On Sat, 2009-05-02 at 22:40 -0700, david@...g.hm wrote:
> On Sun, 3 May 2009, Willy Tarreau wrote:
> 
> > On Sat, May 02, 2009 at 09:33:35AM +0200, Bart Van Assche wrote:
> >> On Fri, May 1, 2009 at 10:59 AM, Andrew Morton
> >> <akpm@...ux-foundation.org> wrote:
> >>> On Thu, 30 Apr 2009 13:26:36 +0200 Philipp Reisner <philipp.reisner@...bit.com> wrote:
> >>>
> >>>> This is a repost of DRBD
> >>>
> >>> Is it being used anywhere for anything?  If so, where and what?
> >>
> >> One popular application is to run iSCSI and HA software on top of DRBD
> >> in order to build a highly available iSCSI storage target.
> >
> > Confirmed, I have several customers who're doing exactly that.
> 
> I will also say that there are a lot of us out here who would have a use 
> for DRDB in our HA setups, but have held off implementing it specificly 
> because it's not yet in the upstream kernel.

Actually, that's not a particularly strong reason because we already
have an in-kernel replicator that has much of the functionality of drbd
that you could use.  The main reason for wanting drbd in kernel is that
it has a *current* user base.

Both the in kernel md/nbd and drbd do sync and async replication with
primary side bitmaps.  The main differences are:

      * md/nbd can do 1 to N replication,
      * drbd can do active/active replication (useful for cluster
        filesystems)
      * The chunk size of the md/nbd is tunable
      * With the updated nbd-tools, current md/nbd can do point in time
        rollback on transaction logged secondaries (a BCS requirement)
      * drbd manages the mirror state explicitly, md/nbd needs a user
        space helper

And probably a few others I forget.

James


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