Re: iodepth and synchronous ioengines ("pitfall")

[Jens Axboe <jaxboe@fusionio.com>]

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.

-- 
Jens Axboe