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

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Message-Id: <1241457851.3315.41.camel@mulgrave.int.hansenpartnership.com>
Date:	Mon, 04 May 2009 17:24:11 +0000
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Philipp Reisner <philipp.reisner@...bit.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

On Mon, 2009-05-04 at 10:28 +0200, Philipp Reisner wrote:
> On Sunday 03 May 2009 16:45:25 James Bottomley wrote:
> > On Sun, 2009-05-03 at 07:36 -0700, david@...g.hm wrote:
> > > On Sun, 3 May 2009, James Bottomley wrote:
> > > > 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.
> > >
> > > one very big one:
> > >
> > > DRDB has better support for dealing with split brain situations and
> > > recovering from them.
> >
> > I don't really think so.  The decision about which (or if a) node should
> > be killed lies with the HA harness outside of the province of the
> > replication.
> >
> > One could argue that the symmetric active mode of drbd allows both nodes
> > to continue rather than having the harness make a kill decision about
> > one.  However, if they both alter the same data, you get an
> > irreconcilable data corruption fault which, one can argue, is directly
> > counter to HA principles and so allowing drbd continuation is arguably
> > the wrong thing to do.
> >
> 
> When you do asynchronous replication, how do you ensure that implicit
> write-after-write dependencies in the stream of writes you get from
> the file system above, are not violated on the secondary ?

Are you telling me drbd doesn't currently do this?

The way nbd does it (in the updated tools is to use DIRECT_IO and
fsync).

> There might be a disk scheduler on the secondary.

There usually is a disk scheduler ... you just have to take the required
action to persuade it to preserve ordering ... a simplistic way of doing
this is to switch to the noop scheduler.

James


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