Arch Linux

Archibald-X

Member

Registered: 2013-12-12

Posts: 8

Udev rules and ssd

On a machine that uses multiple disks, the main one of which is an ssd, I have followed the instructions for creating a udev rule to improve its performance as shown here, https://wiki.archlinux.org/index.php/Solid_State_Drives , in the section entitled "Using udev for one device or HDD/SSD mixed environment". However, after I reboot and check the results, it does not show any change. I switched it in the udev rule from cfq to noop, but cfq still shows after rebooting:

$ cat /sys/block/sda/queue/scheduler

noop deadline [cfq]

So, I was wondering whether there may be another cause for this rule getting blocked? Currently, it is the only one I have made for udev.

teateawhy

Member

From: GER

Registered: 2012-03-05

Posts: 1,138

Website

Re: Udev rules and ssd

Maybe your SSD drive is not correctly registered as non-rotational device with rotational=="0". You should also check the journal, where a possible error in the udev rule would be logged by udev.

lsblk -o NAME,ROTA

Last edited by teateawhy (2013-12-19 19:52:52)

Archibald-X

Member

Registered: 2013-12-12

Posts: 8

Re: Udev rules and ssd

Thank you, teateawhy. Sure enough, running "lsblk -o NAME,ROTA" returned #1 in ROTA column. Even so, "smartctl -a /dev/sda" indicates otherwise, and I know it is an SSD. I haven't a clue as to why it would not have registered correctly. Should I simply do "echo 0 > /sys/block/sda/queue/rotational", reboot and see whether it takes, or is there somehow more to making it so?

Archibald-X

Member

Registered: 2013-12-12

Posts: 8

Re: Udev rules and ssd

Out of curiosity, and honestly in my anxiousness to get this moving in the right direction, I did some more research on this subject and found some interesting information here, https://lwn.net/Articles/408428/ . One of the points made in it provide what I believe is an affirmative answer to my last question in regard to making a change to the value in the '/rotational' folder:

  " There are various ways of detecting SSDs in the hardware, but they don't always work, especially with the lower-quality devices. So the block layer exports a flag under

       /sys/block/<device>/queue/rotational

  which can be used to override the system's notion of what kind of storage device it is dealing with."

Consequently, I am going to manually change the value from 1 to 0 in order "to override the system's notion", which appears to have erred in its assessment of my ssd drive, which, moreover, is hardly one I would consider "lower quality"! Even 'smartctl' has enough sense to see that much. 

Archibald-X

Member

Registered: 2013-12-12

Posts: 8

Re: Udev rules and ssd

All righty then, I have a dilemma. I changed the value of '/sys/block/sda/queue/rotational' from 1 to 0 in an attempt to override the system's notion" of what it has, since this is an ssd drive, not magnetic disks, and I added the udev rule in accordance with the instructions here, https://wiki.archlinux.org/index.php/Solid_State_Drives , as noted above. I even manually issued the command "echo noop > /sys/block/sda/queue/scheduler". However, after rebooting, I again looked at the results from 'lsblk -o NAME,ROTA', and I saw that it still shows "1" for sda. Likewise, I get the following results:

$ cat /sys/block/sda/queue/scheduler

noop deadline [cfq]

Therefore, my udev rule did not seem to make any difference, nor did my manual change to the value of '/rotational'.  So, hmmm(?), now I am as puzzled as ever as to why this is not sinking in. Any ideas would be welcome. I want to change the scheduler to noop for this disk only, and I thought the udev rule would do so. Apparently, something is overriding that decision.

Last edited by Archibald-X (2013-12-20 19:02:43)

teateawhy

Member

From: GER

Registered: 2012-03-05

Posts: 1,138

Website

Re: Udev rules and ssd

Use the script from the archwiki that identifies drives by device ID.
https://wiki.archlinux.org/index.php/SS … devices.29
I used this script before switching to the udev method.

Last edited by teateawhy (2013-12-20 19:54:33)

Archibald-X

Member

Registered: 2013-12-12

Posts: 8

Re: Udev rules and ssd

Yes, indeed, the script surely did work    for indicating here:

$ cat /sys/block/sda/queue/scheduler

[noop] deadline cfq

Still, the value shows as "1" here:  '/sys/block/sda/queue/rotational'.  I suppose the first one is indicative of what the system is actually doing, but I do want to run another test to confirm that result since the other one indicates differently. Thanks, again, for the help with this problem, teateawhy.  Take care.