Maybe there is also another middle-ground solution. I'll try to sketch
it out:
. use sectors instead of time
. impose a penalty to each thread in proportion to the distance between
its disk requests
. reduce the maximum budget of each thread as a function of this seek
penalty so as to prevent the thread from stealing more than a given time
slice (the simple mechanism to limit per-thread budget is already
implemented in bfq).
By doing so, both fairness and time isolation should be guaranteed.
Finally, this policy should be safe in that, given the maximum time used
by a seeky thread to consume its maximum budget on a reference disk, the
time used on any faster disk should be shorter.
Does it seem reasonable?
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