On 03/26/2012 01:59 PM, Martin van Wilderen - JDN BV wrote: > Hi List, > > I have a question about KVM disk performance. We are using Openstack Nova on > three machines. These machines have a SSD drive with a dd write performance > of about 130mb/s > > Within the instance the write performance is down to about 5 mb/s. When using > the allocate trick (dd zero to disk before newfs) we get a performance of 20 > mb/s.
Interesting. I presume this performance difference is when growing the allocation. I.E. without the "dd allocate trick", writes to new files = 5MB/s, but old files = 20MB/s ? What file did you do the `dd` on exactly and what size was it? I could do a patch to nova to do something like: fallocate --version || exit 0 fallocate -l $SIZE "$DISKIMG" || { rm -f "$DISKIMG"; exit 1; } But only when I get a better understand of your setup. Note it would need to be optional too, akin to a memory overcommit like flag. Note the above change would also give the benefit of immediate feedback of ENOSPC > > Things a have tried but don't give any extra results are: > - Settings the disklayout from qcow2 to raw I'm a bit surprised that had no effect. > - Settings the cache type in libvirt.xml (writeback, writethrough, none) > - Switching KSM on and off. > - Tested with different guests OS, Linux, FreeBSD, Windows. > > Is there someone who had some extra info i can check? Or are there more > people with this issue? > > Snippet from libvirt.xml > <driver type='qcow2'/ cache='writeback'/> > <source file='/var/lib/nova/instances/instance-00000113/disk'/> > <target dev='vda' bus='virtio'/> I'll let others more knowledgeable comment on general virt IO overheads. cheers, 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