virsh snapshot-create too slow (kvm, qcow2, savevm)

Bug #741887 reported by Martin Kopta on 2011-03-24
This bug affects 5 people
Affects Status Importance Assigned to Milestone
qemu-kvm (Ubuntu)

Bug Description

# time virsh snapshot-create 1

* Taking snapshot of a running KVM virtual machine

Domain snapshot 1300983161 created
real 4m46.994s
user 0m0.000s
sys 0m0.010s

Expected result
* Snapshot taken after few seconds instead of minutes.

* Ubuntu Natty Narwhal upgraded from Lucid and Meerkat, fully updated.

* Stock natty packages of libvirt and qemu installed (libvirt-bin 0.8.8-1ubuntu5; libvirt0 0.8.8-1ubuntu5; qemu-common 0.14.0+noroms-0ubuntu3; qemu-kvm 0.14.0+noroms-0ubuntu3).

* Virtual machine disk format is qcow2 (debian 5 installed)
image: /storage/debian.qcow2
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 1.2G
cluster_size: 65536
Snapshot list:
1 snap01 48M 2011-03-24 09:46:33 00:00:58.899
2 1300979368 58M 2011-03-24 11:09:28 00:01:03.589
3 1300983161 57M 2011-03-24 12:12:41 00:00:51.905

* qcow2 disk is stored on ext4 filesystem, without RAID or LVM or any special setup.

* running guest VM takes about 40M RAM from inside, from outside 576M are given to that machine

* host has fast dual-core pentium cpu with virtualization support, around 8G of RAM and 7200rpm harddrive (dd from urandom to file gives about 20M/s)

* running processes: sshd, atd (empty), crond (empty), libvirtd, tmux, bash, rsyslogd, upstart-socket-bridge, udevd, dnsmasq, iotop (python)

* networking is done by bridging and bonding

Detail description

* Under root, command 'virsh create-snapshot 1' is issued on booted and running KVM machine with debian inside.

* After about four minutes, the process is done.

* 'iotop' shows two 'kvm' processes reading/writing to disk. First one has IO around 1500 K/s, second one has around 400 K/s. That takes about three minutes. Then first process grabs about 3 M/s of IO and suddenly dissapears (1-2 sec). Then second process does about 7.5 M/s of IO for around a 1-2 minutes.

* Snapshot is successfuly created and is usable for reverting or extracting.

* Pretty much the same behaviour occurs when command 'savevm' is issued directly from qemu monitor, without using libvirt at all (actually, virsh snapshot-create just calls 'savevm' to the monitor socket).

* This behaviour was observed on lucid, meerkat, natty and even with git version of libvirt (f44bfb7fb978c9313ce050a1c4149bf04aa0a670). Also slowsave packages from gave this issue.

Thank you for helping to solve this issue!

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: libvirt-bin 0.8.8-1ubuntu5
ProcVersionSignature: Ubuntu 2.6.38-7.38-server 2.6.38
Uname: Linux 2.6.38-7-server x86_64
Architecture: amd64
Date: Thu Mar 24 12:19:41 2011
InstallationMedia: Ubuntu-Server 10.04.2 LTS "Lucid Lynx" - Release amd64 (20110211.1)
SourcePackage: libvirt
UpgradeStatus: No upgrade log present (probably fresh install)

Martin Kopta (martin-kopta) wrote :
description: updated
tags: added: kvm libvirt qcow2 qemu savevm snapshot virsh
removed: apport-bug
Changed in libvirt (Ubuntu):
status: New → Confirmed
importance: Undecided → Medium
affects: libvirt (Ubuntu) → qemu-kvm (Ubuntu)
Serge Hallyn (serge-hallyn) wrote :

Yup, I can definately reproduce this.

Serge Hallyn (serge-hallyn) wrote :

The current upstream qemu.git from git://
also has the slow savevm. However, it's loadvm takes only a few seconds.

savevm _is_ slow, because it's writing to a qcow2 file with full (meta)data allocation which is terrible slow since 0.13 (and 0.12.5) unless you use cache=unsafe. It's the same slowdown as observed with default cache mode when performing an operating system install into a freshly created qcow2 - it may take several hours. To verify, run `iostat -dkx 5' and see how busy (the last column) your disk is during the save - I suspect it'll be about 100%.

Serge Hallyn (serge-hallyn) wrote :

Confirmed that doing

  kvm -drive file=lucid.img,cache=unsafe,index=0,boot=on -m 512M -smp 2 -vnc :1 -monitor stdio

and doing 'savevm savevm5'

takes about 2 seconds.

So, for fast savevm, 'cache=unsafe' is the workaround. Shoudl this bug then be marked invalid, or 'wontfix'?

Martin Kopta (martin-kopta) wrote :

I confirm that without 'cache' option, I have got from iostat those result while doing 'savevm'

Device: sda
rrqm/s: 0.00
wrqm/s: 316.00
r/s: 0.00
w/s: 94.80
rkB/s: 0.00
wkB/s: 1541.60
avgrq-sz: 32.52
avgqu-sz: 0.98
await: 10.32
svctm: 10.10
%util: 95.76

I also confirm, that when option 'cache=unsafe' is used, snapshot (from qemu monitor) is done as quickly as it should (few seconds).

I am not sure if this is a solution or workaround or just a closer description of a bug. describes option 'cache'. When I use that (cache="none") it spits out:

error: Failed to create domain from vm.xml
error: internal error process exited while connecting to monitor: kvm: -drive file=/home/dum8d0g/vms/deb.qcow2,if=none,id=drive-ide0-0-0,boot=on,format=qcow2,cache=none: could not open disk image /home/dum8d0g/vms/deb.qcow2: Invalid argument

When that option is removed, domain is created successfuly. I guess I have another bugreport to fill.

So, for me, the issue is somehow solved from the qemu side. I think, this could be marked as wontfix.

Kevin Wolf (kwolf-redhat) wrote :

In qemu 0.14 cache=writeback and cache=none are expected to perform well. The default cache=writethrough is a very conservative setting which is slow by design. I'm pretty sure that it has always been slow, even before 0.12.5.

I think that the specific problem with savevm may be related to the VM state being saved in too small chunks. With cache=writethrough this will hurt most.

edison (sudison) wrote :

I had posted a patch to fix the issue before:(, saving memory state is time consuming, which may takes several minutes.

Serge Hallyn (serge-hallyn) wrote :


if you want to push such a patch, please do it through upstream, since it is actually a new feature.

I'm going to mark this 'wontfix' (as I thought I had done before), rather than invalid, though the latter still sounds accurate as well.

Changed in qemu-kvm (Ubuntu):
status: Confirmed → Won't Fix
Cinquero (cinquero) wrote :

Cool. Writes about 9 times the data of the actual snapshot size.

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