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Date: Mon, 07 Dec 2009 07:43:53 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.
References: <4B1943A0.7030509@codemonkey.ws>
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	<20091207103128.GA26970@shareable.org>
	<20091207104517.GJ24530@redhat.com>
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: "Richard W.M. Jones" <rjones@redhat.com>, qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>

Daniel P. Berrange wrote:
> That doesn't work in the case of setting up a clustered filesystem
> shared between guests. That requires that the disk be opened writable,
> but with a shared (F_RDLOCK) lock.
>   

If you'd like data corruption :-)

qemu -drive file=/dev/hda,lock=shared   # clustered file system
qemu -drive file=/dev/hda,lock=shared   # clustered file system

qemu -drive file=/dev/hda,lock=shared -snapshot  # i have no awareness 
of the last two.

The 3rd invocation is totally fine.  /dev/hda is shared (but the 
expectation is read-only).

Better to stick with on/off.  That gives much easier to understand 
semantics.

Regards,

Anthony Liguori