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Date: Sat, 05 Dec 2009 19:31:05 +0200
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.
References: <20091204165301.GA4167@amd.home.annexia.org>	<4B1943A0.7030509@codemonkey.ws>	<20091204215517.GA5938@amd.home.annexia.org>
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: "Richard W.M. Jones" <rjones@redhat.com>, qemu-devel@nongnu.org

On 12/05/2009 12:29 AM, Anthony Liguori wrote:
>
> Well disk sharing isn't actually bad as long as it's raw or a physical 
> device.
>
> For qcow2, it's very complicated by backing files because we really 
> need to express the concept of a read-write lock.
>
> That is, as long as the guests are opening the file read only, you 
> should allow many readers.  However, you should prevent anyone from 
> opening with write permission.  Likewise, writes should prevent future 
> reads.
>

Well shared/exclusive or read/write locks support exactly that.

For non-raw storage we should take the locks unconditionally, since 
there is no useful way to share such images opened for write access.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.