Qcow2 - Performance
Qemu has supported a features called lazy_refcounts from version 1.2. This will noticeably improve the performance of qcow2 disk images when the guest is set to using the writethrough caching mode (which is the default). The tradeoff is that if the guest experiences a sudden power loss, an fsck-like pass will need to be made on the disk image before it can be used again. Luckily, the qemu-img check can now repair qcow2 and QED images with the new -r option.
Setting Cache Type
Below is an example section of an xml configuration of one of my guests where I have set the cache mode to
none (on second line).
I noticed that I didn't have a cache mode set, so it was probably defaulting to writethrough.
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2' cache='none'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
There are four levels of preallocation of qcow2 disk images, listed below in order of least performance to greatest (for guest writes).
My rule of thumb is to use the metadata preallocation as it is quick to deploy small sparse images with a noticeable improvement over having no preallocation at all. If you really need more performance, then go with falloc or full.
Converting Existing Disk Images
If you wish to retrospectively "fix" existing disk images, then you can use the
qemu-img convert command with the relevant options. E.g.
mv disk.qcow2 disk.qcow2.bak
qemu-img convert \
-O qcow2 \
-o lazy_refcounts=on,preallocation=metadata \
# check disk is fine before removing the original
I had recently been suffering from incredibly slow snapshotting of one of my guests. It appears that I may have resolved it by using a script that shuts down the guest before taking the snapshot. In this case the guest had 4GB of RAM, and it appears that saving the memory state was taking up quite a bit of the time.
- Qemu Wiki - Changelog 1.2
- Description of Cache Modes
- JamesCoyle.net - qcow2 Disk Images and Performance
- KVM I/O slowness on RHEL 6
- JamesCoyle.net - qcow2 Physical Size With Different preallocation Settings
- RedHat Docs - Virtualization Tuning and Optimization Guide
- Launchpad - virsh snapshot-create too slow (kvm, qcow2, savevm)
First published: 16th August 2018