[pve-devel] KVM Live Backup performance

Alexandre DERUMIER aderumier at odiso.com
Tue Nov 26 08:20:16 CET 2013

Hi, this is because with new backup,

each new write in the vm during the backup, is copied to backup storage and to the vm.

So, if you have a slow storage, the write to the vm is a the same speed than the backup storage.

This occur only once by block, be doing a full rewrite of the vm disk is the worst case.

Lvm does a snapshot, so backup storage can be slow. New write are only going to the lvm storage.
(But you need to delete the snapshot at the end)

New backup is written sequentially, so sequential write on the backup storage must enough fast to handle write of the vm.


----- Mail original ----- 

De: "Eric Blevins" <eric at netwalk.com> 
À: pve-devel at pve.proxmox.com 
Envoyé: Lundi 25 Novembre 2013 17:31:20 
Objet: Re: [pve-devel] KVM Live Backup performance 

> 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. 

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