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?
On 01/27/2013 04:21 PM, Tom Horsley wrote:
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?
I know there was some fiddling with the cache mode at the qemu level, particularly the default was changed from writethrough to writeback. Maybe something regressed.
Though my understanding was that cache=none should be the faster option. Does that setting produce expected performance? Of course we should still figure out why things slowed down so much with your settings.
- Cole
On Sun, 27 Jan 2013 17:24:29 -0500 Cole Robinson wrote:
Though my understanding was that cache=none should be the faster option. Does that setting produce expected performance? Of course we should still figure out why things slowed down so much with your settings.
Maybe it had something to do with just copying the .xml files from f17.
When I started testing, once I cycled through all the cache modes and booted the machine to run a disk benchmark in each mode, I started seeing the same performance as before. (Or maybe there was something else slowing the VM down and it just seemed like a disk problem during my updates).
The writeback mode still seems slightly faster than none.
On Sun, Jan 27, 2013 at 05:24:29PM -0500, Cole Robinson wrote:
On 01/27/2013 04:21 PM, Tom Horsley wrote:
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?
I know there was some fiddling with the cache mode at the qemu level, particularly the default was changed from writethrough to writeback. Maybe something regressed.
Though my understanding was that cache=none should be the faster option. Does that setting produce expected performance? Of course we should still figure out why things slowed down so much with your settings.
No: the problem with cache=none is that the host does no caching at all, which means that the only memory available for caching is the guest memory, and that is typically smaller than free host memory and therefore less effective as a cache.
Anthony has a good explanation of how filesystem-level changes now mean that other cache modes are safe (whereas they were not before):
http://lists.gnu.org/archive/html/qemu-devel/2012-02/msg02689.html
Rich.
On 01/27/2013 05:40 PM, Richard W.M. Jones wrote:
Anthony has a good explanation of how filesystem-level changes now mean that other cache modes are safe (whereas they were not before):
I assume that cache='none' is still required for shared virtual disks (on LVM), right?
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
Il 28/01/2013 11:01, Kevin Wolf ha scritto:
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.
Yes, cache=unsafe will override it.
Paolo
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Cole Robinson -
Ian Pilcher -
Kevin Wolf -
Paolo Bonzini -
Richard W.M. Jones -
Tom Horsley