Strange disk performance figures

Five_Star · 28 Jan 2009 at 12:33

Hi, got a new server at work, the disk configuration is 4x146GB SAS 10k in RAID 10.
The server is being used to host virtual machines

The first benchmark is a read test inside a virtualised server 2008 x64 on the OS disk

The second benchmark is a read test from a separate, empty virtual hard disk

Why is the start of the first read test so slow, or I suppose why is the end of the test that fast, its faster than I was expecting.

The PERC 6/i RAID card is set to write-back cache and adaptive read ahead.

Five_Star · 28 Jan 2009 at 17:22

Think I may have got this figured out, have to post full details tomorrow. something to do with the virtual disk :s

Rroff · 28 Jan 2009 at 17:30

Yah doing disc tests from inside a VM isn't exactly ideal

cache and burst performance can be way off too.

Five_Star · 29 Jan 2009 at 11:31

The answer, after some more tests is down to the type of virtual hard disk used. By default you get a dynamically expanding hard disk when creating a new VM. When I re-installed on a fixed size virtual hard disk the results were as expected.

Hope this helps someone save an afternoon scratching their head