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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive
Date: Tue, 11 May 2010 23:11:12 +0100
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To: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, armbru@redhat.com, Alexander Graf <agraf@suse.de>, Christoph Hellwig <hch@lst.de>

> > 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