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

[Nanook]

> 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.

     I don't know about 6.0 because I went from 5.something to 6.2, then 6.3
and tuned was installed by default but not with those profiles.  With CentOS
6.2, I manually tuned things, played with read-ahead sizes, etc, until I got
them optimized.  Your milage may vary but with a machine with 32GB of RAM, I
got the best performace with a read ahead of 16384 blocks.  I set the I/O
schedualar to deadline to limit latency.  I played with various cache modes and
for most of my applications write-back was okay (i.e., not storing critical
financial data and crashes are few and far between, journalling file systems
are used, so worst case someone might lose a couple minutes work).

     But that said, one weirdness I have run into while playing with options to
get latency to a minimum, I found that if I enabled kernel premption, the
kernel would only use one core.  Is there anyway to have a preemptive kernel on
a multicore machine and still use all the cores?