Am 27.01.2013 22:21, schrieb Tom Horsley: > Back around fedora 16 I tried all the different virtio > cache modes and experimentally discovered that > cache=none was nearly infinitely faster than cache=default > and cache=writeback was even a little faster than cache=none. > So I set all my VMs to cache=writeback. > > I've been copying my virtual machine defs to each new > fedora since then, but now it seems to be dog slow > with disk IO again. > > I'm planning to experiment with different settings again, > but I have to wait for my Windows XP virtual machine > to finish doing a windows update which may take all night > at the rate it is writing to disk :-(. > > Just thought I'd ask if there is some known difference > in the virtio modes in fedora 18?
One thing that is relatively new, and might not have been in F17, is that virtio-blk automagically enables a writethrough mode if the guest doesn't advertise that it supports flushes. If this is the case, you actually get cache=directsync instead of cache=none, which obviously hurts performance, but ensures that your image doesn't get corrupted on a crash. I don't think there is an option to override this magic, except maybe cache=unsafe. Kevin _______________________________________________ virt mailing list [email protected] https://admin.fedoraproject.org/mailman/listinfo/virt
