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Just say you change the queue_depth on a hdisk with chdev –P. This updates the devices ODM information only, not its running configuration. The new value will take effect next time I reboot the system. So now I have a different queue_depth in the ODM compared to the devices current running config (in the kernel).
What if I forget that I’ve made this change to the ODM and forget to reboot the system for many months? Someone complains of an I/O performance issue....I check the queue_depths and find they appear to be set appropriately but I still see disk queue full conditions on my hdisks. But have I rebooted since changing the values?
How do I know if the ODM matches the devices running configuration?
For example, I start with a queue_depth of 3, which is confirmed by looking at lsattr (ODM) and kdb (running config) output:
# lsattr -El hdisk6 -a queue_depth
queue_depth 3 Queue DEPTH True
# echo scsidisk hdisk6 | kdb | grep queue_depth
ushort queue_depth = 0x3; < In Hex.
Now I change the queue_depth using chdev –P i.e. only updating the ODM.
# chdev -l hdisk6 -a queue_depth=256 -P
hdisk6 changed
# lsattr -El hdisk6 -a queue_depth
queue_depth 256 Queue DEPTH True
kdb reports that the disks running configuration still has a queue_depth of 3.
# echo scsidisk hdisk6 | kdb | grep queue_depth
ushort queue_depth = 0x3;
Now if I varyoff the VG and change the disk queue_depth, both lsattr (ODM) and kdb (the running config) show the same value:
# umount /test
# varyoffvg testvg
# chdev -l hdisk6 -a queue_depth=256
hdisk6 changed
# varyonvg testvg
# mount /test
# lsattr -El hdisk6 -a queue_depth
queue_depth 256 Queue DEPTH True
# echo scsidisk hdisk6 | kdb | grep queue_depth
ushort queue_depth = 0x100; < In Hex = Dec 256.
# echo "ibase=16 ; 100" | bc
256
This is one way of checking you’ve rebooted since you changed your queue_depth attributes. I’ve tried this on AIX 6.1 and 7.1 only.
http://publib.boulder.ibm.com/infocenter/aix/v7r1/topic/com.ibm.aix.kdb/doc/kdb/kdb_pdf.pdf
