[dm-devel] clustered snapshots

[Mikulas Patocka <mpatocka redhat com>]

Hi

I uploaded my test clustered snapshots to 
http://people.redhat.com/mpatocka/patches/kernel/clustered-snapshots-preview/

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.

TODO:
- 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

Mikulas