Re: iodepth and synchronous ioengines ("pitfall")

[Jens Axboe <jaxboe@fusionio.com>]

On 2010-12-02 19:59, Jens Axboe wrote:
> On 2010-12-01 22:38, Sebastian Kayser wrote:
>> Hi,
>>
>> I just stumbled into a glaring pitfall when playing with iodepth=X for
>> the first time. Was using ioengine=sync (default), increased iodepth > 1
>> and wondered briefly why my results didn't change. Thinking about it,
>> this made perfect sense and the "IO depths" distribution in the result
>> summary even pointed me to it.
>>
>> Nevertheless, it might help others to avoid this alltogether if the man
>> page paragraph on iodepth would include a small heads up / reference to
>> ioengines. Example patch attached, not quite sure about the wording for
>> the verify_async aspect.
> 
> It's a good idea. Something else to keep in mind is that even with async
> engines, you can run into this issue. Say in Linux and not setting
> direct=1, the buffered IO will still be sync. So I think I'll add some
> wording as well to have the user keep an eye on the achieved IO depths
> and not just assume that it's running with a depth of X for iodepth=X.

http://git.kernel.dk/?p=fio.git;a=commitdiff;h=ee72ca09d4657f6a805c113d9bc43428f4b28a89

-- 
Jens Axboe