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
