[CentOS-virt] CentOS 6 kvm disk write performance

Julian price centos.org at julianprice.org.uk
Sat Aug 11 18:26:26 EDT 2012

 >> 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' 


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