Combination Disk Scheduler improperly favors local drive

[ylee@…]

I have 16 filesystems in Storage Groups for recordings from my five physical tuners; one filesystem is local to the backend, and the others are remote and are mounted on the backend by CIFS. I wish to have all 16 filesystems treated equally when mythbackend chooses which one to use, with the only criteria being 1) free space and 2) whether the filesystem is being used already. If five recordings begin at once the backend should select the five filesystems with the most free space. To achieve this, I chose the Combination disk scheduler (In the above scenario Balanced Free Space puts all five recordings on the single filesystem with the most free space), and manually inserted SGweightLocalStarting = 0 to have the backend treat local and remote filesystems equally.

When I start a single recording, the disk scheduler still favors the local filesystem even if a remote one has more free space and the calculated weights for all 16 filesystems are equal. (See the attached mythbackend log extract with -v none,schedule,file.) My experience matches ​Jon Larson's report from 2008, back when Combination was the only available setting.

[ylee@…]

mythbackend.log extract with -v none,file,record

[ylee@…]

Patch for 0.25. It alters the balanced I/O disk scheduler to use absolute free space as a tiebreaker if, and only if, weights are equal.

[Yeechang Lee <ylee@…>]

In 8876503c53e612c2663578b2a7750333d9dca208/mythtv:

Allow the Disk I/O Storage Group scheduler to fallback to free space

The "Balanced Disk I/O" Storage Group scheduler was indeterminate when
the weights of two filesystems were equal. This commit makes that
decision more determinate by allowing the scheduler to fall back to using
the amount of free space on the filesystems to make the decision when
two filesystems have the same weight.

Closes #10946.

Signed-off-by: Chris Pinkham <cpinkham@…>