[Xen-users] Perfectly consistent zero-downtime backup

[Niccolò Belli <darkbasic@xxxxxxxxxxxxxxx>]

As far as I know the only way to get a *fully consistent* backup of a 
xen VM is shutting down the virtual machine, creating an LVM snapshot 
and starting the VM again. Unfortunately the downtime is non-trivial and 
you need to shutdown the machine (which may not be a desired behaviour).
Another alternative is saving the machine state, creating an LVM 
snapshot and restoring the machine. You can get a fully consistent 
backup saving the machine state (basically the ram dump) alongside with 
the disk image. Saving and restoring the vm takes a while, but if you 
use ramfs to store the dump you can get nearly-zero down time. 
Unfortunately this kind of backup is far from perfect because you will 
still need to restore the machine to get a consistent state and it may 
be non-trivial if you have to do it in another machine (for example 
because of a dom0 failure).
I thought a third, better way which has the benefits of both previous 
options:
- save the machine state in ramfs
- create a read/*write* LVM snapshot
- restore the machine state from ramfs
- you have a copy of the machine state in ramfs and a vm snapshot thanks 
to LVM, so you can strip attached networks from the config in the head 
of the state dump, change the vm name and point the vm disk to the 
read/*write* LVM snapshot
- xm restore the backup using the machine state you just edited (you can 
run the backup alongside with the original machine because you changed 
the name, stripped the networks and pointed the disk to the read/*write* 
snapshot)
- shutdown the backup VM
- do a fully consistent backup from the lvm snapshot
- remove the lvm snapshot

What do you think about it? Is it feasible?

Cheers,
Niccolò
--
http://www.linuxsystems.it

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