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" <e...@netwalk.com> À: pve-devel@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. _______________________________________________ pve-devel mailing list pve-devel@pve.proxmox.com http://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-devel _______________________________________________ pve-devel mailing list pve-devel@pve.proxmox.com http://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-devel
