Re: [Xen-users] Backup domU

[Andy Smith <andy@xxxxxxxxxxxxxx>]

Hi Greg,

On Wed, Jul 20, 2011 at 09:59:59AM -0600, Greg Woods wrote:
> The /var/log/messages file on the guest shows that the sync did
> occur.  But if I run 'file' on the guest image from the snapshot,
> it still says that the ext3 file system "needs journal recovery".

The filesystem journal will always need recovery unless the
filesystem was cleanly unmounted (by the guest).

> So I am concerned that I may still be getting a semi-corrupted
> image. Is there a way to be certain that the image is clean? Is
> there a way to force the guest to run the journal as well as
> syncing?

Not without shutting it all down cleanly.

As others have pointed out, there may well also be applications in
your guest holding data in their memory that they haven't yet
flushed to disk; sync isn't going to make them do it. Think about
relational databases and the like.

In any backup strategy (for virtualised hosts or those running on
bare metal) if you want it to be the best it can be then you need to
take applications into account. For example for a relational
database this may mean periodically taking a lock and dumping out
the data to a file which is later backed up by your regular backup
process. Such a dump file would have a consistent point-in-time view
of your data, unlike trying to back up the raw data files of the
database.

As the operator of hardware with virtualised guests on it you may
not have that level of insight into just what applications the
guests are running, in which case the administrator of the guest
itself needs to care about that. Your scheme of snapshotting the
filesystem may be the best you can offer without knowing what
applications are running.

It is worth considering running a backup scheme for virtual hosts
that is the same as for any other host, just for the win of being
able to generalise the process.

Cheers,
Andy

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