KVM I/O slowness on RHEL 6

Written by Gionatan Danti on . Posted in Virtualization

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Conclusions

First of all, don't let me wrong: I'm very exited about KVM and libvirt progresses. Now we have not only a very robust hypervisor, but also some critical paravirtualized drivers, a good graphical interface and excellent host / guest remote management capabilities. I would publicly thanks all the talented guys involved in the realization of these great and important projects – thank you boys!

However, it's a shame that the current virt-manager GUI interface don't permit to perform metadata preallocation on QCOW2 image format, as this image is much more feature-rich than the RAW one. Moreover, I would like to see not only the guest creation wizard, but all the guest editing windows to always default to no cache policy for virtual disk, but it is a secondary problem: it is not so difficult to manually change a parameter...

The first problem – no metadata preallocation on QCOW2 – is way more serious, as it can not be overcomed without resort to the command line. This problem should really be corrected as soon as possible. In the meantime, you can use the workaround described above, and remember to always check your virtual disk caching policy – don't use the “default” or “write-through” settings.

I hope than this article can help you to get the max from the very good KVM, libvirt and related projects.

Comments   

 
#1 lawrence 2012-04-27 00:05
very helpful.

thanks.
 
 
#2 Josh 2012-05-10 15:37
This is a GREAT article. However... splitting it up into 8 pages makes it more annoying to use. I would understand doing this, if you had advertising on each page and wanted to derive advertising revenue. But as it stands I've copied all 8 pages into an OpenOffice document so I can turn it into a PDF and file it with the rest of my "useful blog posts".
 
 
#3 Gionatan Danti 2012-05-10 17:01
Quoting Josh:
This is a GREAT article. However... splitting it up into 8 pages makes it more annoying to use. I would understand doing this, if you had advertising on each page and wanted to derive advertising revenue. But as it stands I've copied all 8 pages into an OpenOffice document so I can turn it into a PDF and file it with the rest of my "useful blog posts".


Hi Josh, you are right: while a single page view would be desiderable, the only manner to cover domain's costs is through advertising.

Feel free to store a your personal copy of the article in whatever format you want.

Regads.
 
 
#4 Morrizor 2012-05-16 18:46
Thanks Gionatan, very useful!
 
 
#5 Arturs 2012-06-03 20:36
Hy, it very helpful!
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
 
 
#6 Siauhoo 2012-10-06 09:58
It seem that raw images doesn't support preallocation. if so why there is raw + preallocation in this chart ?
 
 
#7 hakl 2013-12-08 14:51
And where was the big tip which you told on first page?
 
 
#8 Gionatan Danti 2013-12-16 17:56
Quoting hakl:
And where was the big tip which you told on first page?


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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