[CentOS-virt] disk i/o benchmarking inside vm

Mon Oct 10 18:43:14 UTC 2011
Dennis Jacobfeuerborn <dennisml at conversis.de>

On 10/10/2011 08:27 PM, Nehemiah wrote:
> but thats what I'm asking you to investigate. All you see is a symptom,
> more data may clarify the situation. Weren't you taking data from both
> inside the guests an on the host's disks. It was kind of unclear.

The guest number are the ones that are not consistent with each other or 
across restarts of the guests:

guest 1: 120 seeks/s
guest 2: 220 seeks/s

"hdparm -t" shows:
guest 1: 100 MB/s
guest 2: 160 MB/s

The host numbers are the ones of the physical drives:
/dev/sdb: 75 seeks/s
/dev/sdc: 75 seeks/s

hdparm -t:
/dev/sdb: 140 MB/s
/dev/sdc: 140 MB/s

The physical numbers are consistent and what I would expect to see from the 
sata drives.
The guests are minimal centos 6 installations so after booting they have 
virtually no processes running that could influence the benchmarks in any 
significant way. The hosts system is installed on /dev/sda so /dev/sd(b|c) 
are not influenced by the host system either.
The entire setup is arranged to make benchmarking mostly reliable.
If there were minor temporary fluctuations I would blame some external 
process but differences of almost 100% that are consistent for the lifetime 
of the virtual machine do not fit such a scenario.