[linux-lvm] ANNOUNCE: an experimental implementation of snapshot merging
Mikulas Patocka
mpatocka at redhat.com
Thu Jun 5 15:01:15 UTC 2008
On Wed, 4 Jun 2008, Steeve McCauley wrote:
> Is this change meant to add cluster support for merging? I had
> been under the assumption that merging was already a feature of
> snapshots.
My implementation has nothing to do with clustering.
I think there were some userspace programs for off-line merging of
inactive snapshots. My implementation works in kernel and can be used
while the snapshot is mounted.
> What is the difference between the current implementation using
> lvremove (which I assumed was where the COW blocks were merged back
> into the original volume) and lvconvert as below? Why was this
> functionality not kept in lvremove?
lvremove drops the snapshot and forgets its content. lvconvert -M copies
the snapshot to the origin (I'm now working on to let it automatically
drop the snapshot when the merging finishes).
Mikulas
> Mikulas Patocka wrote:
>> Hi
>>
>> Here I release the first experimental implementation of snapshot merging.
>> Merging allows you to copy data in snapshot back to the origin device. Once
>> merging starts, it runs on background. When the merging finishes (you see
>> "0%" with "lvs" command), you should remove the merging snapshot with
>> lvremove command.
>>
>> The userspace will still be changed, the kernel is already finished (there
>> are no known bugs in the kernel, I'm curious about the unknown ones :)
>>
>> Merging is initiated with "lvconvert -M vg/lv_snapshot" command. When you
>> initiate merging, make sure that the origin device is not mounted. Snapshot
>> can be mounted while it's merging is initiated.
>>
>> During merging, aby reads and writes to the origin device are identical to
>> accesses to the merging snapshots.
>>
>> There may be multiple snapshots while one of them is being merged ---
>> exceptions in other snapshots are being allocated and there snapshots are
>> kept stable.
>>
>> Don't try to concurrently merge more than one snapshot (the kernel refuses
>> to do it, the userspace tools don't know about it --- the userspace will be
>> rewritten anyway).
>>
>> The patches are at:
>> http://people.redhat.com/mpatocka/patches/
>> The patches are for 2.6.26-rc4
>>
>> (besides merging, there are some other bugfixes, I made for the kernel)
>>
>> Mikulas
>>
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>
> --
> Steeve McCauley
> Director of Research and Development
> Rackable Systems
> 407 McGill Street, Suite 1002
> Montreal, Quebec, H2Y 2G3 (Canada)
> Tel: 514-288-3343
> http://www.rackable.com
> :wq
>
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