IO depth reported by Fio/blktrace/iowatcher

["Georg Schönberger" <gschoenberger@thomas-krenn.com>]

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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 =)

Best regards, Georg

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