Re: [Xen-users] A simple backup

[Nick Anderson <nick@xxxxxxxxxxxx>]

Nick Anderson wrote:
> 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.
Does that sound right? I guess I am really interested how lvm snapshots 
are thought of as not safe when a domain is paused and you have the 
memory state.
--
Nick Anderson <nick@xxxxxxxxxxxx>
http://www.cmdln.org
http://www.anders0n.net

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