There may be having situation like using luns from two different vendors which may recommend a different queue depths ,In such case you can change this value for individual devices dynamically if the device driver supports it.
# cat < new-depth-value > /sys/block/<sdxx>/device/queue_depth
If the device driver doesn't support you will get the below mentioned message
# echo 1024 > /sys/block/sdx/device/queue_depth
bash: /sys/block/sdx/device/queue_depth: Permission denied
