[Xen-users] Snapshotting LVM backed guests from dom0

[chris <tknchris@xxxxxxxxx>]

Just looking for some feedback from other people who do this. I know
its not a good "backup" method but "crash consistent" images have been
very useful for me in disaster situations just to get OS running
quickly then restore data from a data backup. My typical setup is to
put the LV in snapshot mode while guest is running then dd the data to
a backup file which is on a NFS mount point. The thing that seems to
be happening is that the VM's performance gets pretty poor during the
time the copy is happening. My guesses at why this was happening were:

1.   dom0 having equal weight to the other 4 guests on the box and
somehow hogging cpu time
2.   lack of QoS on the IO side / dom0 hogging IO
3.   process priorities in dom0
4.   NFS overhead

For each of these items I tried to adjust things to see if it improved.

1.   Tried increasing dom0 weight to 4x the other VM's.
2.   Saw pasi mentioning dm-ioband a few times and think this might
address IO scheduling but haven't tried it yet.
3.   Tried nice-ing the dd to lowest priority and qemu-dm to highest
4.   Changing destination to a local

Changing the things above didn't really seem to help either alone or
in combination. My setup is Xen 3.2 and Xen 4.0 on dual nehalem
processors, 24GB RAM, RAID 5+0 of WD RE3 1TB disks. The hardware in
the boxes is quite good and there seems to be no noticable difference
between Xen versions. What I'd ideally like to accomplish is to be
able to take the backups with the least possible impact on the running
VM's as possible. I honestly don't care how long the backups take but
I want to avoid just slowing them down to a fixed speed, because it
seems inefficient/hacky. Can anyone share their experiences both good
and bad?

Thanks,
- chris

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