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    SubjectRe: [RFC PATCH] block: Change default IO scheduler to deadline except SATA
    On Tue, Apr 10 2012 at  3:55pm -0400,
    Jens Axboe <axboe@kernel.dk> wrote:

    > On 2012-04-10 21:43, Mike Snitzer wrote:
    > > I'm still missing your position (other than you now wanting to make it
    > > a userspace concern).
    > >
    > > Put differently: why should CFQ still be the default?
    > >
    > > It is pretty widely held that deadline is the more sane default
    > > (multiple distros are now using it, deadline is default for guests,
    > > etc). CFQ has become more niche. The Linux default really should
    > > reflect this.
    > >
    > > The only case where defaulting to CFQ seems to make sense is
    > > rotational SATA (and USB).
    >
    > That's the precisely the reason it should still be the default. The
    > default settings should reflect a good user experience out of the box.
    > Most desktop machines are still using SATA drives. And even those that
    > made the leap to SSD, lots of those are still pretty sucky at high queue
    > depths or without read/write separation. So I'm quite sure the default
    > still makes a lot of sense.

    I agree that a default of CFQ still makes sense for SATA and USB.

    But why can't there be multiple defaults?

    default: deadline
    SATA and USB default: cfq

    > Punt tuning to the server side. If you absolutely want the best
    > performance out of your _particular_ workload, you are expecting and
    > required to tune things anyway. Not just the IO scheduler, but in
    > general. You can't make the same requirements for the desktop.

    Just because server admins are more used to tuning doesn't mean all
    server admins do it -- especially not on first evaluation.

    > As to kernel vs user, I just see little reason for doing it in the
    > kernel if we can put that policy in user space.

    There are distro packages that are shipped to control such knobs,
    e.g. tuned.

    They don't help _at all_ if the user doesn't know about them. Knob
    tuning is tedious on multiple levels. Much like this thread ;)


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