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[dm-devel] New kernel shared snapshots


The new release of shared snapshots can be found here:

It has the interface we talked about on Linux Tag.
So look at it as a last chance to review the interface. And also test it.

To create a snapshot, you have to:
- send a message
- ask for status, get the snapshot id
- suspend (this creates the snapshot, when the filesystem is quiescent)
- resume

Then, you can attach the snapshots by loading "multisnap-snap" target.

If the table line contains argument "sync-snapshots", it synchronizes the 
snapshot list against what is given on the table --- i.e. creates 
snapshots that are on the table but are not in the store and deletes those 
that are not in the store but are on the table.

Other changes:
- make more structures private (move them to dm-multisnap-private.h), so 
  that ABI doesn't change if these are chaged.
- snapshot IDs are treated as strings, not numbers.
- metadata cache size can be specified as an argument to the "mikulas" 
  exception store.
- an argument "preserve-on-error" that says that on error or overflow, 
  writes to the origin should be disabled.
	- if you don't specify this, the origin continues operation, but 
	snapshots are irreversibly damaged by this (use for non-important
	snapshots such as backups)
	- if you specify this, the data are always preserved, the whole
	thing just halts on error (user when snapshots contain some
	important data.
- fixed two bad bugs in dm-bufio.

Known bugs:
- that read-vs-realloc race (that I already fixed in the old snapshots a 
year ago) is present there. I'll fix it, but it isn't so trivial as in 
unshared snapshots.


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