Default use of caching on raw volumes

[Dustin Kirkland]

Serge E. Hallyn <serge.hallyn at canonical.com> wrote:
> Quoting Nikolai K. Bochev (n.bochev at grandstarco.com):
>> Now the question is - did the ubuntu team do some testing with this ?
>
> I don't know about Dustin.  I myself haven't really played with the
> caching options.

I have, only a bit though.  Mostly to make an informed default in the
TestDrive wrapper for KVM.

I tested and verified this part of the kvm(1) manpage:
 * http://manpages.ubuntu.com/kvm

           Some block drivers perform badly with cache=writethrough, most
           notably, qcow2.  If performance is more important than correctness,
           cache=writeback should be used with qcow2.

For TestDrive, "performance" is more important than "correctness"
since most TestDrive VMs are just throwaway VMs, and all use qcow2.

I have not conducted any deeper analysis, though.  Sorry.

:-Dustin