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Re: [dm-devel] [RFC] dm snapshot based asynchronous replication

On Apr 13, 2009, at 10:29 PM, FUJITA Tomonori wrote:

On Mon, 13 Apr 2009 23:08:20 +0200 (CEST)
christophe varoqui free fr wrote:

dm developers,

I'm in need of an asynchronous replication method for active/ passive clusters using direct-attached disks. Please advise if you know tools (other than drdb an rsync) to do the job. (How goes dm- replicator btw ?).



An asynchronous remote replication based on snapshots is one of
reasons why I worked a new dm snapshot implementation:


Jonathan Brassow has been working on it (Jonathan, Thanks!):


If you add the interface to get the delta between two snapshots, then
you can implement a daemon in user space that does an asynchronous
remote replication.

The exception store API is waiting for agk to push upstream. The 'shared' (aka tomonori/phillips) exception store module is working and ready for testing. I am working on the LVM patches to allow people to use the shared exception store, but this will take me a while. (The cluster-aware exception store is ready for testing as well.)

Regarding the solution you are talking about... I'm not sure how you will get the information out about the deltas between the snapshots. One method I see working would be to use device-mapper's 'message' and 'wait/status' interface. The API already has the ability to pass on messages to the exception stores below, which are free to implement their own features. Still, it would be nice if there was a cleaner way to do this....

Of course, this dialog assumes you've hit upon the right solution to your problem. There may be another way... Direct-attached disks is what is complicating the problem. You could use 'rgmanager' if the devices where shared. You could also use a network block device driver (gnbd, etc) and then use mirroring, but this isn't much different than DRBD.



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