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