On 18-02-15 21:00, Logan Barfield wrote: > Our current deployment is KVM with Ceph RBD primary storage. We have > rbd_cache enabled, and use "cache=none" in Qemu by default. > > I've been running some tests to try to figure out why our write speeds > with FreeBSD are significantly lower than Linux. I was testing both > RBD and local SSD storage, with various cache configurations. Out of > all of them the only one that performed close to our standard Linux > images was local SSD, Qemu cache=writeback, FreeBSD gpt journal > enabled. >
I think the main problem is that FreeBSD uses ide/scsi and Linux uses VirtIO. Have you tried running FreeBSD 10 with the OS type set to Ubuntu for example? > I've been reading on various lists the reasons and risks for > cache=none vs cache=writeback: > - cache=none: Safer for live migration > - cache=writeback: Ceph RBD docs claim that this is required for data > integrity when using rbd_cache > > From what I can tell performance is generally the same with both, > except in the case of FreeBSD. > > What is the current line of thinking on this? Should be using 'none' > or 'writeback' with RBD by default? Is 'writeback' considered safe > for live migration? > None should still be ok, since librbd does the caching. It's not Qemu who does the caching. Wido
