Re: [Qemu-devel] Default cache mode

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Jun 29, 2011 at 12:59 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> I'm not entirely sure if I should suggest writeback or none as the new
> default, but I think it could make sense to change it.

I agree that cache=none is safe and fast with correct guests and local
disks.  It is beaten by cache=writeback in cases where the host has a
lot of buffer cache to play with and the guest has very little - for
example when running qemu without a -m argument.  If I was running
qemu-system-arm I might like cache=writeback.  If I was virtualizing
x86 servers I might like cache=none.

cache=none as a default - which some management tools inherit - would
be reasonable.  Folks using command-line qemu can always tweak to use
cache=writeback or even cache=unsafe if performance matters but data
integrity does not.

Whether it is safe to transition to cache=none or not depends on what
broken guests are still widely deployed.  Does CentOS 5 flush the disk
cache?

Stefan