Re: [Qemu-devel] linux aio and cache mode

[Kevin Wolf <kwolf@redhat.com>]

Am 26.09.2012 01:22, schrieb ching:
> On 09/25/2012 09:33 PM, Kevin Wolf wrote:
>> 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
>>
> 
> is it possible to check the open mode of file and whether it is using aio at runtime?

You can attach strace and look for open/pwritev/iosubmit and friends. I
did that yesterday with qemu-io and didn't see any iosubmit for 'qemu-io
-k -t unsafe'.

Kevin