[fedora-virt] Virt Disk Performance

[Richard W.M. Jones]

On Mon, Oct 31, 2011 at 10:18:50AM +0100, Kevin Wolf wrote:
> Am 30.10.2011 21:59, schrieb Richard W.M. Jones:
> >> I've pulled most of the information you requested. See above. I
> >> don't see any specific cache settings on either machine. Is there
> >> somewhere I would see the default setting on the machine?
> > 
> > I'm out of the loop on what the default caching policy is these days.
> > Hopefully libvirt is at least choosing a safe one.
> 
> cache=writethrough is the default. And indeed this is the safest option,
> and at the same time by far the slowest option. I would expect that you
> get much better write performance with cache=none.
> 
> Note however that older guests OSes cannot deal correctly with disks
> that have a volatile write cache, like cache=none emulates. This means
> that in case of a host crash and with some bad luck, your guest might
> experience file system corruption.
> 
> Recent guest OSes are safe in this respect and can be used with
> cache=none or cache=writeback in order to improve performance without
> any such risk.

Just FYI, here is how libvirt XML types are mapped to qemu types,
found by examining the libvirt code:

  libvirt          old qemu      modern qemu
  
  none             cache=off     cache=none
  writeback        cache=on      cache=writeback
  writethrough     cache=off     cache=writethrough
  directsync       cache=off     cache=directsync (if supported by qemu)
  unsafe           cache=off     cache=unsafe (if supported by qemu)

I tested this to see what libvirt would actually use with my fairly
recent qemu-kvm, and what sort of timings I would get:

  libvirt          MB/s          modern qemu command line
  
  (no setting)     29            (nothing)
  none             39            cache=none
  writeback        24*           cache=writeback
  writethrough     28            cache=writethrough
  directsync       [did not work for me -- "unknown disk cache mode"]
  unsafe           [did not work for me -- "unknown disk cache mode"]

* = large variability in this result

What's also interesting is that performance today with no cache
setting is slower than it was yesterday, even though I'm not running
anything significantly different on my laptop.

Kevin, I'm using 'dd ... conv=fsync' for these tests.  What's a good,
reliable method for testing read and write speeds?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://et.redhat.com/~rjones/virt-df/