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Date: Wed, 13 Oct 2010 09:11:49 -0500
From: Anthony Liguori <aliguori@linux.vnet.ibm.com>
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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


>> > >
>> > >That means we can maintain the physical size without introducing
>> > >additional fsync()s in the allocation path.  Since we're already
>> > >writing out the header anyway, the write operation is basically
>> > >free too.
>> >
>> >  I don't see how it is free.  It's an extra write.  The good news is
>> >  that it's very easy to amortize.
>>
>> We only need to update the header field on disk when we're already
>> updating the header, so it's not even an extra write operation.
>
> 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.

Regards,

Anthony Liguori