[pve-devel] KVM Live Backup performance

[Eric Blevins]

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

I have not tested writes yet and doubt I will have time to get to that 
this week.

On a positive note, CPU intensive tasks seems to be slightly faster 
using KVM Live Backup vs LVM Snapshot.
Used: sysbench --test=cpu --cpu-max-prime=50000 run

Averages from 10 trials:
No backup: 92.57103s
KVM Live Backup: 93.21844s
LVM Snapshot Backup: 93.51216s

Eric