Re: [linux-lvm] ANNOUNCE: an experimental implementation of snapshot merging

From: Steeve McCauley <steeve@rackable.com>
To: LVM general discussion and development <linux-lvm@redhat.com>
Subject: Re: [linux-lvm] ANNOUNCE: an experimental implementation of snapshot merging
Date: Wed, 04 Jun 2008 10:01:54 -0400	[thread overview]
Message-ID: <4846A052.10607@rackable.com> (raw)
In-Reply-To: <Pine.LNX.4.64.0806031516380.20514@engineering.redhat.com>

Is this change meant to add cluster support for merging?  I had
been under the assumption that merging was already a feature of
snapshots.

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?

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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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

-- 
Steeve McCauley
Director of Research and Development
Rackable Systems
407 McGill Street, Suite 1002
Montreal, Quebec, H2Y 2G3 (Canada)
Tel: 514-288-3343
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:wq