Re: IO depth reported by Fio/blktrace/iowatcher

[Jens Axboe <axboe@kernel.dk>]

On 2012-10-03 09:02, Georg Schönberger wrote:
> Good Morning,
> 
> I have a short question about the used io depth reported by Fio/blktrace/iowatcher:
> If I am starting a test:
> # blktrace -d /dev/sde -o hdd &
> # fio --rw=read --name=wd --bs=1024k --direct=1 --filename=/dev/sde --offset=0 --runtime=300 --ioengine=libaio --iodepth=4
> [...]
>   IO depths    : 1=0.1%, 2=0.1%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
>      submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
>      complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
> [...]
> As seen above Fio is reporting 100% io depth 4. In contrast to that blktrace and iowatcher (cf. attached figure) revealing the following io depths (9 and 7):
> # blkparse hdd.blktrace.8
> [...]
> CPU8 (hdd):
>  Reads Queued:        9072,     4644MiB	 Writes Queued:           0,        0KiB
>  Read Dispatches:     9070,     4643MiB	 Write Dispatches:        0,        0KiB
>  Reads Requeued:         0		 Writes Requeued:         0
>  Reads Completed:     9072,     4644MiB	 Writes Completed:        0,        0KiB
>  Read Merges:            0,        0KiB	 Write Merges:            0,        0KiB
>  Read depth:             9
> [...]
> # iowatcher -t hdd.blktrace.8 -o wd.svg
> (showing an io depth of 7)
> 
> Where is this divergence concerning the io depths coming from? A short explanation would be great =)

You are using a relatively large block size (1024k) and that is why.
That will be broken into 512kb chunks usually, effectively almost
doubling the queue depth seen on the device side.

Fio reports the queue depth the way it sees it, on the submitting
application side. That may or may not be identical to what the device
sees. It could be higher, if the scheduler is throttling fio. Or it
could be lower, as in this case.

-- 
Jens Axboe