On Sat, Aug 11, 2012 at 5:26 PM, Julian price <centos.org@xxxxxxxxxxxxxxxxxx> 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 > > > > > _______________________________________________ > CentOS-virt mailing list > CentOS-virt@xxxxxxxxxx > http://lists.centos.org/mailman/listinfo/centos-virt 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 _______________________________________________ CentOS-virt mailing list CentOS-virt@xxxxxxxxxx http://lists.centos.org/mailman/listinfo/centos-virt
