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Re: [Qemu-discuss] backup procedure validation

From: Raphael Bauduin
Subject: Re: [Qemu-discuss] backup procedure validation
Date: Wed, 18 Sep 2013 08:43:04 +0200

Hi Phil,

thanks for your feedback. My intention is indeed not to use the commands listed as is in a script.
I wanted to validate the procedure of mounting the disk as a nbd snapshot and rsyncing that.

I'm asking because this backup procedure seems so simple and easy to set up compared to qemu-img snapshots that I'm surprised it's not more documented or discussed.


On Tue, Sep 17, 2013 at 5:47 PM, Phil Ehrens <address@hidden> wrote:
I assume that you intend to script this and run it via cron?

Running a series of commands without testing their return
values and intelligently parsing stdout/stderr is a formula
for disaster.

I would run rsync a second time and parse the messages
returned to be 100% certain of success. rsync is the tool
for validating rsync.

You should do a simple test to make certain that the
mount succeeded so that the sync goes to the mount and
NOT to /mnt/src on the root filesystem.

I would say that >90% of the snapshot/backup scripts that I
see have fatal flaws. The result is usually either a failure
to snapshot/backup, or filling up of the wrong filesystem.

Raphael Bauduin wrote:
> Hi,
> I found out qem-nbd can mount a snapshot, but this feature is not much
> documented or discussed (or I didn't find the good resources), so I'd like
> to have some confirmation before I use it for my backups.
> Here is the sequence of actions I would take:
>  stop domain
>  qemu-nbd --connect /dev/nbd0 --read-only --snapshot /path/to/file.qcow2
>  start domain
>  mount /dev/nbd0p1 /mnt/src
>  rsync -a /mnt/src /mnt/dest
>  umount /mnt/src
>  qemu-nbd --disconnect /dev/nbd0
> With these actions, can I be sure that
> - my backup of the filesystem is correct and coherent  ?
> - the domain can continue to work normally despite the nbd snapshot ?
> - when the nbd is disconnected, its snapshot is removed ?
> Thanks
> Raphaël

Phil Ehrens <address@hidden> |              mm
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