Has something happened to virtio cache modes?

[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?

[Cole Robinson]

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

[Tom Horsley]

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.

[Richard W.M. Jones]

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.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

[Ian Pilcher]

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?

-- 
========================================================================
Ian Pilcher                                         arequipeno(a)gmail.com
Sometimes there's nothing left to do but crash and burn...or die trying.
========================================================================

[Kevin Wolf]

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

[Paolo Bonzini]

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