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Date: Wed, 13 Oct 2010 09:08:17 -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: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Christoph Hellwig <hch@lst.de>, Avi Kivity <avi@redhat.com>, qemu-devel@nongnu.org

On 10/13/2010 09:07 AM, Stefan Hajnoczi wrote:
>
>>> 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.
>    

Because we're already writing out the sector that contains that field in 
the header.

Regards,

Anthony Liguori

> Stefan
>