[CentOS-virt] CentOS 6 kvm disk write performance

Trey Dockendorf treydock at gmail.com
Sat Aug 11 18:56:26 EDT 2012

On Sat, Aug 11, 2012 at 5:26 PM, Julian price
<centos.org at julianprice.org.uk> wrote:
>  >> physical disk position shouldn't have such a marked effect should it?
> Nanook wrote:
>  > Actually the physical disk position can make a HUGE difference.
> Thank you Nanook for your explanation. I think you're right. It looks
> like the variation in performance is not due to LVM, but the position on
> the disk. If the volumes' write performance figures are rearranged in
> the order of their position on the disk, they are roughly in line.
> The root disk is first, so it is best. Then there are no figures for the
> swap file. The archive volume is roughly half the remaining space, so
> I'd expect it's performance to be closer to root. But archive is quite
> full, so maybe the bonnie++ test files went towards the end, near the
> other VM partitions. The last volume to be allocated has the worst
> performance. It all makes sense!
> So, when calculating the disk performance hit from using a VM, or the
> effectiveness of VM disk optimisations, the benchmark is the performance
> of the volume that hosts the corresponding virtual disk, not the root disk.
>> For those reasons I always try to put swap and I/O critical stuff, like
>> swap, at the beginning of the drive and loath partitioning software that thinks
>> it's smarter and puts things where it wants.
> And when there are many VMs, each with its own root disk, swap file,
> temp disk, there's a challenge! Well, at least I can try to ensure that
> the busiest VM gets the first logical volume.
> All this shows the importance of basing decisions on evidence from
> performance tests on your own servers, not assumptions & other peoples'
> findings.
> Thanks,
> Julian
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I did a "study", more like a quick experiment a few months back to
determine best combination between Preallocation and Caching and
posted my results here,
http://itscblog.tamu.edu/improve-disk-io-performance-in-kvm/.  It's
not very scientific as it was just done by timing the same kickstart
CentOS install 4 times with varying parameters for Preallocation and
Caching.  I also only used qcow2 images, which do not perform as well
as raw.

- Trey

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