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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 13:15:08 +0100
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To: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, armbru@redhat.com, Christoph Hellwig <hch@lst.de>, Alexander Graf <agraf@suse.de>

> > What about another cache=... value instead of adding more options? I'm
> > quite sure you'll only ever need this with writeback caching. So we
> > could have cache=none|writethrough|writeback|wb-noflush or something
> > like that.

I agree.
 
> The cache option really isn't too useful.  There's a matrix of 3x2
> possible I/O modes for the posix backend, and right now we only expose
> two of them.  I think we really should expose all of them, split into
> two options:
> 
>                                       caching
> 
>                     | normal             | O_DIRECT           |
> 
>             --------+--------------------+--------------------+
>             none    | -                  | -                  |
> integrity   O_DSYNC | cache=writeback    | -                  |
>             fsync   | cache=writethrough | cache=none         |
>             --------+--------------------+--------------------+

I think this table is misleading, and from a user perspective there are only 4 
interesting combinations:

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.

cache=writeback
  Reads and writes are cached. Guest flushes are honoured. Note that this may 
include a guest visible knob that allows the write cache to be disabled. If 
the guest requests the cache be disabled then this should act the same as 
cache=writethrough. Closest to how real hardware works.

cache=always (or a more scary name like cache=lie to defend against idiots)
  Reads and writes are cached. Guest flushes are ignored.  Useful for dumb 
guests in non-critical environments.

IMO the other boxes in your table (disable cache but require guest flushes) 
don't make any sense.

Paul