[pve-devel] KVM Live Backup performance

[Dietmar Maurer]

> 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

Why does that show a "decrease in IO read performance"?
I guess the dd inside the VM is much faster with live backup?