On Tue, 2017-01-03 at 10:39 +0100, Paolo Valente wrote:
> > Il giorno 03 gen 2017, alle ore 09:17, Bart Van Assche <Bart.VanAss
> > che@xxxxxxxxxxx> ha scritto:
> >
> > * Since BFQ has been designed for hard disks and since the approach
> > in BFQ for handling deceptive idleness reduces bandwidth, what
> > scheduling algorithm to use for storage media that do not have any
> > moving parts (SSDs and MMC).
>
> I would really like to have the opportunity to debunk this false
> myth. BFQ is optimized for rotational as well as non-rotational
> device. BFQ does not keep up only if IOPS go beyond ~50k. And I'm
> already working on this limitation, but, as agreed with Jens, the
> priority for the moment is pushing BFQ as it is.
Hello Paolo,
A quote from the section "Service Properties" from a paper from 2010
about BFQ: "In this section we report the service properties of BFQ, in
both sector (bandwidth distribution) and time domains. [ ... ] As a
consequence, one would set Twait as high as possible to include as many
applications as possible. However, the value of Twait has an important
impact on the disk throughput. It may provide significant boosting in
presence of deceptive idleness, but only if its value is in the order
of the seek and rotational latencies [1], namely a few milliseconds. In
contrast, higher values may cause progressive performance degradation,
as the disk may be left idle for too long."
Did I understand correctly that BFQ uses disk idling to improve
sequential I/O throughput for harddisks? If so, my question is why you
think that disk idling will not reduce throughput for SSDs since disk
idling keeps the queue depth low while SSDs perform best for workloads
with a high queue depth?
Thanks,
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