Sometimes you’ll want to change these parameters – usually to allow you to tune the device for performance. To list the attributes available for a device, use the lsattr command.
The most common flags here are:
-l to point to the logical device
-E for the effective values in use.
It is helpful to display the header for each field, too. You can do that using the –H flag. Here’s a list of the attributes for a virtual SCSI disk using “lsattr –EHl hdisk2”:
attribute value description user_settable PCM PCM/friend/vscsi Path Control Module False algorithm fail_over Algorithm True hcheck_cmd test_unit_rdy Health Check Command True hcheck_interval 0 Health Check Interval True hcheck_mode nonactive Health Check Mode True max_transfer 0x40000 Maximum TRANSFER Size True pvid 00c49d54db682c060000000000000000 Physical volume identifier False queue_depth 24 Queue DEPTH True reserve_policy no_reserve Reserve Policy True
To display just the one attribute, you can specify it with the –a flag. Here’s how to see the queue depth (number of concurrent I/Os allowed on a disk).
lsattr -EHlhdisk2 -a queue_depth attribute value description user_settable queue_depth 24 Queue DEPTH True
If you can see that this attribute is “user settable” (it can be changed by you), then it’s sometimes helpful to know the available values. You can do that by replacing the –E in the lsattr command with a –R. You’ll need to specify which attribute you want to change. Once again, use the –a flag.
Here are the possible values I can change the queue depth to:
lsattr -Rl hdisk2 –a queue_depth 1...256 (+1)
… which means I could go up to a queue depth of 256. Incidentally, you’d need to change that on the Virtual I/O Server as well, or else the client will be allowed to pump a large number of I/Os through and then the VIOS will be the bottleneck, due to a small queue depth on its disk.
For more information about changing the queue depth, and how this can help with disk performance, see the IBM Techdoc referred to in the resources section at the bottom of this article.
Changing a device’s attribute is usually done with the chdev command.
chdev –l device –a attribute=value
So, here’s how to enable dynamic tracking on a fibre channel adapter.
chdev –l fcs0 –a dyntrk=yes
And if you’re wondering about dynamic tracking and why you’d ever want to change it, see that same Techdoc I just mentioned.
Sometimes you’ll try to change a device and get an error reporting that the device is busy. It will depend on the device but if it’s something that is constantly in use (such as a disk in rootvg), you’ll either have to free it up from use by any processes, or else set a flag on the chdev command which makes the change permanent. The flag is –P
chdev –l fcs0 –a dyntrk=yes -P
I like to think of the –P flag as the “postpone-till-reboot” flag. Once you make the change on the device, it will take effect from the next reboot.
Of course, there are special commands which deal specifically with disks – commands that are covered by the AIX Logical Volume Manager – but for most other devices, I tend to get by with lsattr to list their attributes and chdev to change them.
IBM Techdocs Library: Setting the queue depth for performance tuning
IBM Knowledge Center: List device attributes with the lsattr command
IBM Knowledge Center: Change device characteristics using chdev
Having worked on IBM systems since 1991, Anthony English has seen how AIX and virtualisation can manage all kinds of business environments. He is a well-recognised author in the IT field and he writes about business systems improvement through his blog http://anthonyenglish.com.au. Anthony is based in Sydney, Australia, with his wife and his seven children.