[Bug 741887] [NEW] virsh snapshot-create too slow (kvm, qcow2, savevm)

[dum8d0g]

Public bug reported:

Action
======
# time virsh snapshot-create 1

* Taking snapshot of a running KVM virtual machine

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

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

Environment
===========
* 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:
ID        TAG                 VM SIZE                DATE       VM CLOCK
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
https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/524447 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)
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: libvirt
UpgradeStatus: No upgrade log present (probably fresh install)

** Affects: libvirt (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 kvm libvirt natty qcow2 qemu savevm snapshot virsh

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https://bugs.launchpad.net/bugs/741887

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