Re: [Xen-devel] consistent LVM snapshot of domUs from dom0

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> Just a question about unsafely. I have implemented iscsi and zfs as
> combination with Xen on Linux. Is the only safe procedure:
>
> pause
> memory snapsnot
> [send out disk snapshot]
> resume

As long as you've got a reasonably recent version of Xen, I think that should 
work fine.

I guess you're using a Solaris as a guest and Linux as a host?  I'm not so 
familiar with Solaris or zfs...

However, I can perhaps be a little speculative.  Since zfs is 
transaction-based you probably don't actually *have* to pause the guest 
before you back it up...  If you were to LVM snapshot the VBD from dom0, then 
the resulting snapshot would probably still be a mountable ZFS volume, it 
would just be as if the power had been cut off to the VM at that point.  You 
would lose cached and in-memory data (I guess there might be incomplete 
writes to files, but I'm not even sure that'd be the case).  There'd be no 
guarantee that the data are consistent at the application level, but there's 
no general way to ensure consistency at application level in a running 
machine anyhow.

In summary: you might be able to get away with not taking the memory snapshot, 
based on my understanding of ZFS (which is a bit vague).  You could 
experiment with this, but really it's not going to take long to do a memory 
snapshot too and that seems the safest (works regardless of FS 
implementation) and most complete (won't lose any data, even cached and 
in-memory state).

I hope that helps a bit, sorry for hand-waving so much but I don't have 
specific experience with what you're doing!

Cheercs,
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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