Re: [Xen-users] A simple backup

[Nick Anderson <nick@xxxxxxxxxxxx>]

Mark Williamson wrote:
> This is not really ideal, since it's not always desirable to have to save a 
> domain's memory state.  It would be nicer if you could somehow guarantee a 
> consistent filesystem state on your disk snapshot.  I've been looking into 
> how to do this and I believe it's quite possible to modify the code to 
> support this properly.
Mark, thanks for the reply.
My thought is this. If I xm save it saves the memory state and pauses 
the VM. While the vm is paused I can then snapshot the block device from 
dom0 of the guest vm. Since the domain is paused there is no disk 
activity during the snapshot, and anything that was possibly 
half-written is in the memory state. So I should be able to reliably dd 
the snapshot since when I try to restore I could try restoring the block 
 device state, and use xm restore on the checkpoint to restore the 
memory at the same time. Is this not essentially what xm migrate does?
Can we not be assured that since we have memory state and block state at 
that point in time that the data will be consistent?
.... When I say assured I mean just as assured as any other backup. 
There is always the possibility of flipped bits, I am just thinking that 
having memory state and block state when attempting a restore would be 
very very close if not the same as having turned the domain off and 
copied the block device then turn it back on.
hope that makes sense and there wasnt too much rambling.


--
Nick Anderson <nick@xxxxxxxxxxxx>
http://www.cmdln.org
http://www.anders0n.net

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