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[dm-devel] Re: IO scheduler based IO controller V10

Vivek Goyal wrote, on 10/01/2009 10:57 PM:
Before finishing this mail, will throw a whacky idea in the ring. I was
going through the request based dm-multipath paper. Will it make sense
to implement request based dm-ioband? So basically we implement all the
group scheduling in CFQ and let dm-ioband implement a request function
to take the request and break it back into bios. This way we can keep
all the group control at one place and also meet most of the requirements.

So request based dm-ioband will have a request in hand once that request
has passed group control and prio control. Because dm-ioband is a device
mapper target, one can put it on higher level devices (practically taking
CFQ at higher level device), and provide fairness there. One can also
put it on those SSDs which don't use IO scheduler (this is kind of forcing
them to use the IO scheduler.)

I am sure that will be many issues but one big issue I could think of that
CFQ thinks that there is one device beneath it and dipsatches requests
from one queue (in case of idling) and that would kill parallelism at
higher layer and throughput will suffer on many of the dm/md configurations.


As long as using CFQ, your idea is reasonable for me.  But how about for
other IO schedulers?  In my understanding, one of the keys to guarantee
group isolation in your patch is to have per-group IO scheduler internal
queue even with as, deadline, and noop scheduler.  I think this is
great idea, and to implement generic code for all IO schedulers was
concluded when we had so many IO scheduler specific proposals.
If we will still need per-group IO scheduler internal queues with
request-based dm-ioband, we have to modify elevator layer.  It seems
out of scope of dm.
I might miss something...

IKEDA, Munehiro
  NEC Corporation of America
    m-ikeda ds jp nec com

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