Re: [Xen-users] A simple backup

[Nico Kadel-Garcia <nkadel@xxxxxxxxx>]

Nick Anderson wrote:
> 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.
Any operation that has not paged disk operations out to the disk image 
have their changes stashed in RAM: this makes databases  It's bad enough 
when it's  a text file in the middle of being edited, but when it's a 
database like Oracle or MySQL or even db4, you're vulnerable to some 
real trouble because their 'atomic operations' won't be.
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