Re: [Xen-users] Backup domU image at runtime

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Andrea Monti wrote:

> I think that taking a backup of a virtual disk while it is used is 
> even worse: if you yank the power cord and you are using a journaled 
> file system, the file system will probably recover by itself.
> If you backup a virtual disk while it is used you are taking an 
> inconsistent image of the journal and an inconsistent (corrupted) 
> image of the files!
If you do a "file copy" then yes, the whole thing will be 
inconsistent. If you use something like an LVM snapshot then it would 
be a "point in time" copy and equivalent to yanking the power cord.
> I think that the risk will always be high: you cannot be guaranteed 
> that no process will write any file during the backup
See above about snapshots.

> and you have no way to check if the backup is working (the os will 
> see the filesystem as ok, but application data could still be 
> corrupted)
Indeed. Which is why I specifically said it depends heavily on what 
the guest is doing. Databases in particular would be a problem as 
most DB engines do a lot of their own caching.
Simplest advice on such backups is ... DON'T !
Less simple is ... DON'T unless you understand the nuances enough to 
determine that it's safe for **your** situation.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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