[pve-devel] KVM Live Backup performance

[Michael Rasmussen]

On Fri, 22 Nov 2013 11:41:43 -0500
Eric Blevins <eric at netwalk.com> wrote:

> I have identified one use-case where KVM Live Backup causes a significant decrease in IO read performance.
> 
> Start a KVM Live Backup
> Inside the VM immediately run:
> dd if=/dev/disk_being_backed_up of=/dev/null bs=1M count=8192
> 
> Repeated same test but used LVM snapshot and vmtar:
> lvcreate -L33000M -s -n test-snapshot /dev/vmdisks/vm-108-disk-2
> /usr/lib/qemu-server/vmtar  '/etc/pve/qemu-server/108.conf' 'qemu-server.conf' '/dev/vmdisks/test-snapshot' 'vm-disk'|lzop -o /backup1/dump/backup.tar.lzop
> 
> KVM Live Backup: 120 seconds or more
> LVM Snapshot backup: 55 seconds
> With no backup: 45 seconds
> 
> Even worse was to read from an area far away from where the backup process is reading.
> I started the backup, in the guest I ran:
> dd if=/dev/disk_being_backed_up of=/dev/null bs=1M count=8192 skip=24000
> 
> KVM Live Backup: 298 seconds
> LVM Snapshot Backup: 58 seconds
In a thread on the proxmox forum discussing performance of cheph
(http://forum.proxmox.com/threads/16715-ceph-perfomance-and-latency)
Dietmar replies: "A VM is only a single IO thread"
Could this influence the performance when doing the new KVM live backup
since this backup occurs inside the KVM while the old LVM backup is
taken place outside the KVM?


-- 
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Michael Rasmussen

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