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Re: [Qemu-devel] Issue with snapshot outside qcow2 disk - qemu 0.14.0

From: Jes Sorensen
Subject: Re: [Qemu-devel] Issue with snapshot outside qcow2 disk - qemu 0.14.0
Date: Fri, 11 Mar 2011 07:31:51 +0100
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101209 Fedora/3.1.7-0.35.b3pre.fc14 Thunderbird/3.1.7

On 03/10/11 22:04, Stefan Hajnoczi wrote:
> On Thu, Mar 10, 2011 at 7:57 PM, SAURAV LAHIRI <address@hidden> wrote:
>> The high level use case is that of being able to backup user specified disks 
>> of a VM without having to bring down the VM.
> Excellent, that sounds exactly like Jes is addressing so future
> QEMU/KVM releases will hopefully have the live snapshot/merge
> capability.
>> snapshot_blkdev: Regarding this  I do have a couple of questions.
>> 1. If the snapshot cannot be merged then it could mean that there are 
>> several snapshot files. One readonly  for each of the previous snapshots and 
>> the last one being the active one, which handles all the current writes. 
>> Post backup If do have to restore to a particular snapshot then i would 
>> probably have to copy all the files in the chain and maintain the entire 
>> chain. But would it not affect read performance if several snapshot files 
>> are maintained, particularly if the VM is hosting a database like mysql ? 
>> Could you please clarify.
> If the VM is not running you can use the qemu-img commit command to
> merge the snapshot back down into the base image.  After that you only
> have one image file again and can restart the VM.  Hopefully the
> deltas are small enough that this process is quick.
> In the future a live merge command will take care of this and avoid
> the downtime.

Yep, qemu-img convert should be able to copy it into a single image so
you can delete the chain.


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