Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive

[Paul Brook <paul@codesourcery.com>]

> > Paul Brook wrote:
> > > cache=none:
> > >   No host caching. Reads and writes both go directly to underlying
> > >   storage.
> > > 
> > > Useful to avoid double-caching.
> > > 
> > > cache=writethrough
> > > 
> > >   Reads are cached. Writes go directly to underlying storage.  Useful
> > >   for
> > > 
> > > broken guests that aren't aware of drive caches.
> > 
> > These are misleading descriptions - because cache=none does not push
> > writes down to powerfail-safe storage, while cache=writethrough might.
> 
> If so, then this is a serious bug.

.. though it may be a kernel bug rather that a qemu bug, depending on the 
exact details.  Either way, I consider any mode that inhibits host filesystem 
write cache but not volatile drive cache to be pretty worthless.  Either we 
guaranteed data integrity on completion or we don't.

Paul