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

[Philipp Reisner <philipp.reisner@linbit.com>]

On Sunday 03 May 2009 16:45:25 James Bottomley wrote:
> On Sun, 2009-05-03 at 07:36 -0700, david@lang.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@lang.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@linux-foundation.org> wrote:
> > >>>>> On Thu, 30 Apr 2009 13:26:36 +0200 Philipp Reisner 
<philipp.reisner@linbit.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 ?

There might be a disk scheduler on the secondary.

-Phil
-- 
: Dipl-Ing Philipp Reisner
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