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Date: Tue, 25 Sep 2012 15:33:31 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] linux aio and cache mode
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To: ching <lsching17@gmail.com>
Cc: qemu-devel@nongnu.org

Am 25.09.2012 00:40, schrieb ching:
> On 09/24/2012 08:30 PM, Kevin Wolf wrote:
>> Am 24.09.2012 13:32, schrieb ching:
>>> Hi all,
>>>
>>> My host is qemu-1.1.1 and x64 kernel 3.5.4. The guest is using aio="native"
>>>
>>> I am trying to use unsafe cache mode to boost i/o performance.
>> aio=native requires the image to be opened with O_DIRECT, i.e.
>> cache=none or cache=directsync. If you specify a different cache option,
>> it will silently fall back to aio=threads.
>>
>> Kevin
>>
> 
> will qemu log a entry for the silent fallback?

No, that's why it's silent. :-)

> Reason:
> 
> I am testing sparse image on btrfs with mount option: rw,noatime,space_cache,autodefrag,inode_cache
> 
> i encounter a speed difference (around 2X-3X) between aio=threads,cache=unsafe and aio=native,cache=unsafe
> 
> aio=threads is much faster, i guest there is conflict between "autodefrag" and linux aio

This is odd. The point is that with cache=unsafe it shouldn't even be
using Linux AIO in the first place. I can't see why there would be any
difference between aio=threads and aio=native with cache=unsafe.

Kevin