[LU-715] Value read from qos_threshold_rr file does not match value written

Wed, 9 May 2012 12:17:51 +0000

After I change the value of qos_threshold_rr by writing to /proc/fs/lustre/lov/<fsname>-mdtlov/qos_threshold_rr, the value I read back is usually one less than the value I wrote:

[root]# echo 17 > qos_threshold_rr
[root]# cat qos_threshold_rr
16%

This is not the case for values 0 or 100:

[root]# echo 100 > qos_threshold_rr
[root]# cat qos_threshold_rr
100%
[root]# echo 0 > qos_threshold_rr
[root]# cat qos_threshold_rr
0%

I realize this is an artifact of the way the value is stored. (Value range 0-100 is converted to 0-256 when written, and then converted back when read.) However, it is confusing to write one value and then read back a different value. This could be especially problematic when the value is 1:

[root]# echo 1 > qos_threshold_rr
[root]# cat qos_threshold_rr
0%

Anyone reading a value of 0 would not know whether the value had been set to 0 or 1.

If the behavior of Lustre cannot be modified, then the documentation should probably be updated to explain it.

Whamcloud Community JIRA

[LU-715] Value read from qos_threshold_rr file does not match value written