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Date: Mon, 07 Dec 2009 08:55:37 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.
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To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: Avi Kivity <avi@redhat.com>, qemu-devel@nongnu.org

Richard W.M. Jones wrote:
> So to be clear, the use case is that all the other VMs must be shut
> down, then the VM which wants to commit will upgrade its lock and
> commit, and then all the other VMs will restart?  I agree this should
> avoid corruption, although it sounds like something which is fairly
> unlikely to be done in practice.
>   

Commit is an extremely uncommon use-case.  There's really no correct way 
to use it.  That said, assuming people used it, here would be a 
reasonable flow:

1) user starts up with base.img -> cow.img
2) user makes some changes, realized it would be good for the golden 
master so issues a commit to base.img
3) user does not shut down this machine
4) user tries to start another vm with a base.img -> cow2.img

This is a valid scenario and temporarily promoting to write lock during 
commit allows this scenario without introducing a window for data 
corruption.

Regards,

Anthony Liguori