KVM I/O slowness on RHEL 6

Written by Gionatan Danti on . Posted in Virtualization

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The influence of the virtualized I/O controller

Another important thing to check is the influence of the virtualized I/O controller that is presented to the guest. KVM let you use not only the default IDE virtual controller, but also a new, paravirtualized I/O controller called VirtIO. This virtualized controller promise better speed and less CPU usage.

How it affect the results?

IDE vs VirtIO controller performance

As you can see, the write-through scenario is the most affected one, while with the no-caching and write-back policies it has a lesser effect.

This does not means that the VirtIO is an unimportant project: the scope of this test was only to be sure that it don't comport any I/O slowness. In a following article I will analyze this very promising driver in a much more complete manner.

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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