Re: [Xen-users] About tape drive.

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > Hmmmm.  I generally don't recommend mounting filesystems of running domUs
> > in dom0, even in read-only form: it can confuse the filesystem driver in
> > dom0, potentially leading (in the worst case) to system instability,
> > corrupted data being read for the backup, etc.  I imagine there are also
> > potential security implications (e.g. privileged data ending up in
> > unprivileged files) for the guest's filesystem.
>
> Oh, that's what LVM snapshots are for! To protect the active filesystem
> from anything the Dom0 might do while mounting the LVM snapshot.

Yes, doing an LVM snapshot in dom0 of the guest's disk image will make it 
safely mountable (from the point of view of not upsetting the dom0 or domU 
filesystem code).

It still doesn't solve the problem of making the guest's FS consistent, so 
fsck may be required on the snapshot itself.  I guess how much of a problem 
this is depends on the filesystem.  I imagine that ext3, for instance, will 
generally recover from this pretty well.  If the guest is running another 
filesystem, this might vary (for instance, XFS does not respond well to this 
sort of thing, I think).

But yes, you are right - LVM snapshots are the way to go for consistent guest 
backups.  Not using some kind of snapshotting to back up a live guest would 
be a very bad plan and I forgot to point this out!

> > It's best to backup disk images of cleanly shutdown guests.  Or you could
> > backup the disk of a guest which has been xm save-ed *and* back up the
> > guest's memory image at the same time.  The two together contain all the
> > information needed to consistently read the disk when the guest is
> > resumed, although the disk image can't be mounted on it's own (and, if
> > you do, you won't be able to safely resume the guest afterwards!).
>
> "Cleanly shutting down guests" is begging to cause issues for systems
> without high availability, especially databases.

Absolutely, but I think it's worth making clear that there are potential 
problems with naive approaches to backing up guest data - especially for 
highly available systems.

There should really be better mechanisms in Xen to support efficient, 
consistent backups of guest filesystems without needing a full shutdown.  
Unfortunately, at the moment the infrastructure isn't all there (even though 
most of the required functionality already exists).

Shutdown (or potentially an in-guest backup solution) also has the benefit 
that you can make sure daemons have stopped, saved cached data to files 
appropriately, etc.

> If you're going that route, I recommend "rsnapshot" or plain old
> "rsync". If you're clever, you can do it to the LVM snapshot and speed
> the heck out of the process, because most of the files are already there!

Ah!  So snapshot the guest, then update the snapshot every so often with an 
rsync from the guest?.

Whilst we're on the subject my personal favourite backup system is 
rdiff-backup, which provides compressed, diff-based backups of a filesystem - 
either locally, or over SSH - and provides "time-travel" capabilities for 
getting old versions of files.

Cheers,
Mark

-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

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