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

[ching]

On 09/26/2012 03:51 PM, Kevin Wolf wrote:
> 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
>

The output of strace when using aio=native:

open("XXXXXX", O_RDWR|O_CLOEXEC) = 14


However, i can no longer reproduce the problem the VM image is defragged, it is 
weird.

Anyway, thank you very much for your help.

ching