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Date: Mon, 07 Dec 2009 07:47:39 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.
References: <20091204165301.GA4167@amd.home.annexia.org>
	<20091207103908.GI2271@arachsys.com>
	<4B1D03E0.5080006@codemonkey.ws>
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To: Chris Webb <chris@arachsys.com>
Cc: "Richard W.M. Jones" <rjones@redhat.com>, qemu-devel@nongnu.org

Chris Webb wrote:
> Anthony Liguori <anthony@codemonkey.ws> writes:
>
>   
>> I've resisted this because I'm concerned that if we delay the
>> opening of the file on the destination, it could fail.  That's a
>> very late failure and that makes me uncomfortable as just a work
>> around for NFS.
>>     
>
> I don't know much about NFS's semantics with cache-coherency, but I take it
> there isn't some sort of synchronisation operation that works on a file
> descriptor and could be done instead as the vm starts and stops to avoid
> having to delay the open itself? I agree with you that failing this late
> could make for some rather nasty behaviour compared to the current version.
>   

I think this is a case of spec vs. in practice.  By the NFS spec, 
coherence is only guaranteed as close-after-open.  Let me poke around 
and see if I can find anyone with better info.

Regards,

Anthony Liguori
> Cheers,
>
> Chris.
>