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Date: Tue, 11 May 2010 19:20:20 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive
Message-ID: <20100511182020.GB30446@shareable.org>
References: <1273528310-7051-1-git-send-email-agraf@suse.de>
	<4BE91708.7050307@redhat.com> <20100511105559.GA21363@lst.de>
	<201005111315.08897.paul@codesourcery.com>
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To: Paul Brook <paul@codesourcery.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Alexander Graf <agraf@suse.de>, Christoph Hellwig <hch@lst.de>, qemu-devel@nongnu.org, armbru@redhat.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.

> 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.

cache=unsafe would tell it like it is.

Even non-idiots could be excused for getting the wrong impression from
cache=always.

-- Jamie