[Qemu-discuss] Re: data consistency on LVM snapshots using writeback caching

[Nicolas Sebrecht]

The 14/07/13, Daniel Neugebauer wrote:
> Hi!
> 
> I have some grave generic disk latency issues on the servers I
> virtualized under Linux/Qemu/KVM + virtio_blk. Virtual block devices are
> setup to run raw on LVM logical volumes with cache=none. Whenever some
> writes happen (according to iotop it's only a few kB every few seconds)
> disk latency in VMs goes up to 1.5 seconds. I understand that
> cache=writeback may help but I am unable to find any details about
> whether it is safe to use it in the way I use LVs:
> 
> Backups are currently being created by calling sync inside the guest OS
> and snapshotting the LVs immediately afterwards on the host. The host
> then mounts those snapshots (usually causing journal replay on them) and
> starts saving data from it. Snapshots are discarded afterwards. Another
> side-effect of high disk latency might be that all VM hosts have issues
> releasing those LVM snapshots afterwards (they tend to need a few
> seconds or minutes before they can be deactivated and removed).
> 
> I noticed that a server with hardware RAID + BBU and 256MB write cache
> does not suffer from these issues. However, that's far from what I can
> afford on all other servers, apart from my personal distaste for
> hardware RAIDs.

I wonder how you handle RAID for your LV because in my experience LVM
RAIDs are very slow in comparison to mdadm RAID. I have issues like you
describe for guests running on LVM RAID (not using raw LV, though).

-- 
Nicolas Sebrecht