[CentOS-virt] tuned-adm fixed Windows VM disk write performance on CentOS 6

Julian price centos.org at julianprice.org.uk
Sun Aug 12 08:44:40 EDT 2012

On a 32bit Windows 2008 Server guest VM on a CentOS 5 host, iometer 
reported a disk write speed of 37MB/s

The same VM on a CentOS 6 host reported 0.3MB/s.  i.e. The VM was unusable.

Write performance in a CentOS 6 VM was also much worse, but it was usable.
(See http://lists.centos.org/pipermail/centos-virt/2012-August/002961.html)

With iometer still running in the guest, I installed tuned on the CentOS 
6 host and enabled the virtual-host profile.  iometer showed the average 
disk write speed increasing immediately. A fresh iometer test reported a 
disk write speed of 80MB/s.

I'm not sure if the tuned-adm virtual-host profile was available in 6.0 
- it may be necessary to update to get it.

Installation is quick & easy:
yum install tuned
tuned-adm profile virtual-host

I'm surprised not to see any other posts on this subject.  Maybe there's 
something peculiar about my setup that caused the dreadful write 
performance & tuned-adm happened to fix it.  This problem has been a 
real headache and I'm extremely grateful to Philip Durbin and jimi_c for 
bringing the solution to my attention.

The host server was rebuilt from scratch after a RAID1 simultaneous 
double disk failure (happily my DR strategy worked. Have you tested 
yours recently?).  It appears that CentOS 6 has some different defaults 
(more conservative power saving?).  Maybe upgrades don't change the 
defaults, so this issue only affects new CentOS 6 installs?

The iometer settings were: 1000 sectors, 16 outstanding IOs, access 
specification: 16k, 0%read 0%random
The figures I've reported above are the 'Total MBs per second'
See http://www.iometer.org/doc/downloads.html

In the process of researching the problem, 2 other quick & easy 
optimisations came to light, each adding a few more MB/s:
- Set noatime. See: 

- Disable Windows disk cacheing: See 
On Windows 2008: Computer Mgmt -> Disk Mgmt -> Select Drive (system) -> 
properties -> Policies tab -> uncheck 'Optimize for quick removal'

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