On 03/04/2013 07:54 PM, Pádraig Brady wrote: > On 03/04/2013 07:19 PM, Samuel Winchenbach wrote: >> Hi All, >> >> I have a cluster of three nodes set up. Live migration is accomplished via >> an NFS mount over 10GigE shared by all three nodes. >> >> I have noticed that when a VM is brand new I am getting close to 60MB/s disk >> write when using "dd if=/dev/zero of=file bs=1M count=1k conv=fdatasync". >> after doing this 10s of times the perfomance of the disk seems to drop to >> around 4 - 12 MB/s. >> >> I have also noticed that doing a live-migration causes a similar effect >> immediately. >> >> Here is the virsh xml output of one of my VMs: >> https://gist.github.com/swinchen/397fbe3bb74305064944 >> >> I have read several "tuning" guides and most of the suggestions seems to be >> configured already (cache='none', virtio for network, disk and memballoon). >> >> >> Do you think qcow2 is causing my issue and if so is there a way to boot an >> instance and override the disk format? > > qcow may possibly be the issue. > > You could use raw disk images by setting the > use_cow_images=False nova config option. > The tradeoff there is slower instance start and > increased (but more guaranteed) disk usage. > > Alternatively you could try to fallocate the disk > image before running the perf test like: > # get virt_size > qemu-img info /var/lib/nova/instances/instance-0000002f/disk > fallocate -n -l $virt_size /var/lib/nova/instances/instance-0000002f/disk > > There is also the possibility of adjusting nova > to use preallocation=metadata on the qcow images > (forfeiting CoW in the process), as discussed in "future work" at: > https://blueprints.launchpad.net/nova/+spec/preallocated-images
Just did some testing here, writing in a VM backed by a local file system, using: dd if=/dev/zero of=file bs=1M count=1k conv=notrunc,fdatasync oflag=append I didn't see a degredation after a while, but did see quite different performance depending on the formats used: disk performance outside VM = 120MB/s raw in $instance_dir/ = 105MB/s qcow copy with preallocation=metadata in $instance_dir/ = 100MB/s qcow CoW with fallocate full size in $instance_dir/ = 55MB/s Note perf a bit more stable than without fallocate I didn't test with full host disk where improvements would be more noticeable qcow CoW in $instance_dir/ = 52MB/s qcow CoW in $instance_dir/ backed by qcow with preallocation=metadata in base = 52MB/s thanks, Pádraig. _______________________________________________ Mailing list: https://launchpad.net/~openstack Post to : openstack@lists.launchpad.net Unsubscribe : https://launchpad.net/~openstack More help : https://help.launchpad.net/ListHelp
