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Date: Tue, 22 Apr 2008 09:53:13 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [kvm-devel] [Qemu-devel] Re: [PATCH 1/3] Refactor AIO interface
	to allow other AIO implementations
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To: qemu-devel@nongnu.org, kvm-devel@lists.sourceforge.net, Marcelo Tosatti <mtosatti@redhat.com>

Jamie Lokier wrote:
> Avi Kivity wrote:
>   
>>> And video streaming on some embedded devices with no MMU!  (Due to the
>>> page cache heuristics working poorly with no MMU, sustained reliable
>>> streaming is managed with O_DIRECT and the app managing cache itself
>>> (like a database), and that needs AIO to keep the request queue busy.
>>> At least, that's the theory.)
>>>       
>> Could use threads as well, no?
>>     
>
> Perhaps.  This raises another point about AIO vs. threads:
>
> If I submit sequential O_DIRECT reads with aio_read(), will they enter
> the device read queue in the same order, and reach the disk in that
> order (allowing for reordering when worthwhile by the elevator)?
>   

There's no guarantee that any sort of order will be preserved by AIO 
requests.  The same is true with writes.  This is what fdsync is for, to 
guarantee ordering.

Regards,

Anthony Liguori