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[dm-devel] Re: clustered snapshots

I've just looked at the last group of patches briefly. So far, I like what I see. I especially like the concept of locking locally first, and then switching to cluster. If I add cluster lock caching - this will further speed things up.

I'd prefer to have the UUID that is passed in on the CTR line be more general - leaving the possibility for future features/flags, rather than just plopping an extra argument on the end. I also need to look over the 'reread_exceptions' stuff, as this seems a little light... I assume you didn't just take my patch to allow rereads because by adding an additional function to the API, you simplify some of the logic?

Also, any progress on adding the cluster locking to the your shared snapshots?


On Sep 29, 2009, at 4:39 PM, Mikulas Patocka wrote:


I uploaded a new version there. It has selective re-read and it has
optimized locking --- it does no remote communication if the chunk already
exists, so it should be as efficient as Jon's approach.

Jon, please look at it and review the patches, if you find out that this approach can't work at all, we can drop it and go with your approach. If
it is OK, we can consider adopting it, it looks simpler than making


On Mon, 28 Sep 2009, Mikulas Patocka wrote:


I uploaded my test clustered snapshots to

My patches take different approach from Jon's patches. My patches
basically replace down_write(&s->lock) and up_write(&s->lock) with
clusterized locking.

If there are pending exceptions, the cluster lock must be held while the
local lock is unlocked. The cluster lock is droppen when all pending
exceptions are reallocated and the local lock is dropped.

The patches are based on my & Mike's merging.

These patches are less invasive than Jon's, they area small, they don't change so much logic and most importantly, they leave merging as it is.

Note that it was never tried in a cluster because I don't have a cluster!! So there may be a silly bug that makes it not work at all. The purpose of
the patches is to show different simpler approach to clustering. You
should test it and debug it.

- implement lock caching (Jon's task for his dm-lock module)
- once we implement it, we can implement selective re-read --- i.e. don't reread the exceptions if the cluster lock was not taken by any other node - in a few cases we could optimize it to use readlock or only local lock
- implemented cluster merging


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