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When I'm manipulating over some benchmark tool I/O depth, what exactly am I changing?

Let's say I have SATA disk on /dev/sda. If I run hdparm:

root@linaro-gnome:~# hdparm -I /dev/sda | grep Queue
Queue depth: 32
* Native Command Queueing (NCQ)

If I check I/O scheduler queue depth:

root@linaro-gnome:/# cat /sys/block/sda/queue/nr_requests
128

If I check scsi queue depth:

root@linaro-gnome:/sys/bus/scsi/devices/0:0:0:0/block# ls
sda

root@linaro-gnome:/# cat /sys/bus/scsi/devices/0\:0\:0\:0/queue_depth
31

What am I changing above when manipulating over I/O depth? When someone refers to I/O depth, what is he referring to?

And is there something else I'm missing?

Thanks

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