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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH
Date: Tue, 20 Apr 2010 14:22:58 +0100
References: <20100218222220.GA14847@redhat.com>
	<20100420014635.GE21899@shareable.org>
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To: qemu-devel@nongnu.org
Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, Rusty Russell <rusty@rustcorp.com.au>, virtualization@lists.linux-foundation.org, hch@lst.de

> Does this mean that virtio-blk supports all three combinations?
> 
>    1. FLUSH that isn't a barrier
>    2. FLUSH that is also a barrier
>    3. Barrier that is not a flush
> 
> 1 is good for fsync-like operations;
> 2 is good for journalling-like ordered operations.
> 3 sounds like it doesn't mean a lot as the host cache provides no
> guarantees and has no ordering facility that can be used.

(3) allows the guest to queue overlapping transfers with well defined results.
I have no idea how useful this is in practice, but it's certainly plausible.

Paul