Re: Read I/O starvation with writeback RAID controller

From: Jan Engelhardt
Date: Fri Feb 22 2013 - 15:35:32 EST

On Friday 2013-02-22 20:28, Martin Svec wrote:
> Yes, I've already tried the ROW scheduler. It helped for some low iodepths
> depending on quantum settings but generally didn't solve the problem. I think
> the key issue is that none of the schedulers can throttle I/O according to e.g.
> average request roundtrip time. Shaohua Li is right here:
> -- as long as there's free room in
> device's queue they blindly dispatch requests to it.
> Which is exactly what I see in deadline scheduler fifo queues: There're no read
> requests to be scheduled between writes because all readers are starving. So
> the scheduler keeps dispatching writes using all the remaining capacity of
> device queue. Which in turn worses the read starvation. Bigger queue depth and
> bigger writeback cache means higher chance for read starvation even from a
> single writer.

Sounds just like the bufferbloat problem in networking.
Waiting for codel for the block layer :)
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