KVM I/O slowness on RHEL 6
QCOW2 image format performance
The QCOW2 format is the default QEMU/KVM image format. It has some very interesting features, as compression and encryption, but especially it enable the use of real, file-level snapshots.
But how it performs?
Mmm... without metadata preallocation, it performs very badly. Enable metadata preallocation, stay away from write-through cache and it perform very well.
To better compare it to the RAW format, I made a chart with the no-caching RAW and QCOW2 results:
While without metadata preallocation the QCOW2 format is 5X slower then RAW, with enabled metadata preallocation the two are practically tied. This prove that while RAW format is primarily influenced by caching setting, QCOW2 is much dependent on both the preallocation and caching policies.
Comments
thanks.
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Regads.
I tried myself with default server is centos 5.8 x86_64 is installing for long long time, formating image is very long!
But when i tried cache none on full(full size already allocated) image, its running perfect
Hi, the tip was to preallocate the QCOW2 backing file (as explained in the article) and to _not_ use the writethrough cache setting.
However, newer KVM/Qemu version (starting with those found in CentOS 6.1) greatly improved the usability of non-preallocate d QCOW2 files.
So, if you take the route to go with QCOW2 and you need thin provision, you can avoid to preallocate the backing file. However, remember to avoid the writethough cache.
Regards.
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