Re: [Xen-users] Re: 100% safe way to backup domU: (was Yet another backup)

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > I think you should just be able to backup the device file that you'd
> > usually mount.  Just treat the snapshot's device file as *the* file to
> > backup.
>
> Unfortunately, backing up devices is rife with risks, due to databases
> (which may have things in RAM not yet written to disk but which will
> mess up things that *have* been written to disk if they don't happen),
> and due to disk paging (stuff that to a user looks like it's been
> written to disk may not have actually made it to the disk yet, it may
> not be paged out yet). The sync command helps with the latter, but can't
> do anything about the former.
>
> Thinking about these things is why people who make industrial grade
> backup software get paid.

Yep.  The whole device backup strategy should only be used if you're saving a 
copy of the guest's memory to accompany the drive image (in which case you 
should have all of the dirty buffers that are required to make sense of the 
drive's contents).

The only problem with this is *possibly* that there's a race condition: I 
suspect we can't guarantee that guest disk writes that have been issued by 
the backend have all hit the LVM device at the point where the snapshot is 
taken...  This was also a theoretical problem with live migration so I guess 
it should be fixed in the same way, but I don't know if that's happened yet.

I suspect this might not be an issue in practice.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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