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Date: Wed, 13 Oct 2010 09:53:30 -0500
From: Anthony Liguori <aliguori@linux.vnet.ibm.com>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] Re: [PATCH v2 6/7] qed: Read/write support
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To: Avi Kivity <avi@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Christoph Hellwig <hch@lst.de>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org

On 10/13/2010 09:16 AM, Avi Kivity wrote:
>  On 10/13/2010 04:11 PM, Anthony Liguori wrote:
>>> Why would you ever update the header, apart from relocating L1 for 
>>> some reason?
>>
>>
>> To update the L1/L2 tables clean bit.  That's what prevents a check 
>> in the normal case where you have a clean shutdown.
>
> I see - so you wouldn't update it every allocation, only when the disk 
> has been quiet for a while.

Right, the current plan is to flush the header dirty bit on shutdown or 
whenever there is an explicit flush of the device.  Current that is 
caused by either a guest-initiated flush or a L1 update.  We also plan 
to add a timer-based flush such that a flush is scheduled for some 
period of time (like 5 minutes) after the dirty bit is set.

The end result should be that the only window for requiring a metadata 
scan is if a crash occurs within 5 minutes of a cluster allocation and 
an explicit flush has not occurred for some other reason.

Regards,

Anthony Liguori