N o harm will come to existing VMs if I set "use_cow_images=False" and restart nova-api correct? Just all new VMs will be created with a raw disk backend?
Thanks, Sam On Mon, Mar 4, 2013 at 2:54 PM, Pádraig Brady <p...@draigbrady.com> 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 > > thanks, > Pádraig. >
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