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Re: [dm-devel] [PATCH v2 0/5] dm-replicator: introduce new remote replication target

On Thu, 2009-11-26 at 09:18 -0600, James Bottomley wrote:
> On Thu, 2009-11-26 at 13:29 +0100, heinzm redhat com wrote:
> > From: Heinz Mauelshagen <heinzm redhat com>
> > 
> > 
> > * 2nd version of patch series (dated Oct 23 2009) *
> > 
> > This is a series of 5 patches introducing the device-mapper remote
> > data replication target "dm-replicator" to kernel 2.6.
> > 
> > Userspace support for remote data replication will be in
> > a future LVM2 version.
> > 
> > The target supports disaster recovery by replicating groups of active
> > mapped devices (ie. receiving io from applications) to one or more
> > remote sites to paired groups of equally sized passive block devices
> > (ie. no application access). Synchronous, asynchronous replication
> > (with fallbehind settings) and temporary downtime of transports
> > are supported.
> > 
> > It utilizes a replication log to ensure write ordering fidelity for
> > the whole group of replicated devices, hence allowing for consistent
> > recovery after failover of arbitrary applications
> > (eg. DBMS utilizing N > 1 devices).
> > 
> > In case the replication log runs full, it is capable to fall back
> > to dirty logging utilizing the existing dm-log module, hence keeping
> > track of regions of devices wich need resynchronization after access
> > to the transport returned.
> > 
> > Access logic of the replication log and the site links are implemented
> > as loadable modules, hence allowing for future implementations with
> > different capabilities in terms of additional plugins.
> > 
> > A "ringbuffer" replication log module implements a circular ring buffer
> > store for all writes being processed. Other replication log handlers
> > may follow this one as plugins too.
> > 
> > A "blockdev" site link module implements block devices access to all remote
> > devices, ie. all devices exposed via the Linux block device layer
> > (eg. iSCSI, FC).
> > Again, other eg. network type transport site link handlers may
> > follow as plugins.
> > 
> > Please review for upstream inclusion.
> So having read the above, I don't get what the benefit is over either
> the in-kernel md/nbd ... which does intent logging, or over the pending
> drbd which is fairly similar to md/nbd but also does symmetric active
> replication for clustering.

This solution combines multiple devices into one entity and ensures
write ordering on it as a whole like mentioned above, which is mandatory
to allow for applications utilizing multiple devices being replicated to
recover after a failover (eg. multi device DB).
No other open source solution supports this so far TTBOMK.

It is not limited to 2-3 sites but supports up to 2048, which ain't
practical I know but there's no artifical limit in practical terms.

The design of the device-mapper remote replicator is open to support
active-active with a future replication log type. Code from DRBD may as
well fit into that.

> Since md/nbd implements the writer in userspace, by the way, it already
> has a userspace ringbuffer module that some companies are using in
> commercial products for backup rewind and the like.  It strikes me that
> the userspace approach, since it seems to work well, is a better one
> than an in-kernel approach.

The given ringbuffer log implementation is just an initial example,
which can be replaced by enhanced ones (eg. to support active-active).

Would be subject to analysis if callouts to userspace might help.
Is the userspace implementation capable of journaling multiple devices
or just one, which I assume ?


> James

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