[pve-devel] KVM Live Backup performance

Eric Blevins eric at netwalk.com
Mon Nov 25 17:31:20 CET 2013

> 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.

More information about the pve-devel mailing list