[pve-devel] KVM Live Backup performance

[Eric Blevins]

> I am unable to reproduce that - for me LVM and Live backup are about the same speed.
>
> Can you see the effect if you dump backup output directly to /dev/null?
>
> # /usr/lib/qemu-server/vmtar  '/etc/pve/qemu-server/108.conf' 'qemu-server.conf' '/dev/vmdisks/test-snapshot' 'vm-disk'>/dev/null
>
> # vzdump <VMID> -stdout>/dev/null

There is little difference reading inside the VM when writing to 
/dev/null, 117MB/sec vs 131MB/sec with LVM.

Here is the best way I have to demonstrate the problem, simulate slow 
backup media using cstream.
Slower backup media results in greater IO performance loss in the VM.

vzdump <vmid> -stdout|cstream -t 60000000 > /dev/null

In the VM I ran:
dd if=/dev/vdb of=/dev/null bs=1M count=8192 skip=2400

140MB/sec Limit, 109 MB/s Read speed in VM
120MB/sec Limit,  95.6 MB/s Read speed in VM
100MB/sec Limit,  80.3 MB/s Read speed in VM
  80MB/sec Limit,  65.6 MB/s Read speed in VM
  60MB/sec Limit,  47.2 MB/s Read speed in VM
  40MB/sec Limit,  31.8 MB/s Read speed in VM

With LVM snapshot backups the VM does not suffer performance problems if 
the backup media is slow, the backup simply takes longer.