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

[ching <lsching17@gmail.com>]

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?