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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] Issue with snapshot outside qcow2 disk - qemu 0.14.0
Date: Thu, 10 Mar 2011 21:04:32 +0000

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

> 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.

> 2. I have seen that at times the qemu monitor command is not able to connnect 
> to  the monitor socket as libvirt it seems controls the monitor socket. If I 
> shutdown libvirt then commands like socat is able to connect. But since my 
> current environment does use libvirt, shutting down libvirt is not an option. 
> Is there any way around this ?

New versions of libvirt have a virsh "qemu-monitor-command" command
you can use to send a QEMU monitor command.


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