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    SubjectRe: EXT4-ish "fixes" in UBIFS
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    On Thu, Apr 2, 2009 at 7:05 PM, Trenton D. Adams
    <trenton.d.adams@gmail.com> wrote:
    > On Thu, Apr 2, 2009 at 7:55 PM, David Rees <drees76@gmail.com> wrote:
    >> On Thu, Apr 2, 2009 at 5:28 PM, Trenton D. Adams
    >> <trenton.d.adams@gmail.com> wrote:
    >>> Oh, I should have clarified.  It improves performance under heavy
    >>> load.  Under normal load, mounting without sync is fine.  What I tend
    >>> to do is mount with "remount,rw,sync" when heavy load is starting.
    >>> Then my system goes slowly, but latency is good.  Then, when it's all
    >>> done (say a big compile, or job, or whatever), I remount without sync
    >>> again.
    >>>
    >>> I'm thinking of writing a script that monitors performance, and
    >>> remounts as needed, lol.  WHAT A HACK. hehe.
    >>
    >> All you're doing here is implementing the lowering of dirty data
    >> limits in the VM dynamically based on how long fsyncs take.
    >>
    >> Linus outlined this specific strategy as "the ideal siutation"
    >> somewhere in the depths of "That filesystem thread".
    >
    > I thought he said it was a HORRIBLE solution. :D  I recall him
    > slamming Andrew over it.  Unless you're referring to the kernel
    > actually doing it on the fly.

    Yes - you are correct - doing it in userspace isn't the best place to
    put it - but if you can do it there, the same ideas could then be
    pushed into the kernel and further enhanced.

    >> Look at the new in 2.6.29 dirty*bytes parameters in
    >> Documentation/sysctl/vm.txt for more info.  By lowering those values,
    >> you can effectively turn normal writes into synchronous writes which
    >> will greatly reduce latency of fsync under heavy write load.
    >>
    >> In previous kernels you can tweak dirty_ratio and
    >> dirty_background_ratio, but they don't have the granularity of the new
    >> knobs.  Although if you are talking about just remounting in sync
    >> mode, they may work for you at least as a proof of concept. ;-)
    >
    > dirty_ratio and dirty_background never really had any affect for me.
    > I'll look into the other parameters.  Waiting for the checkout again,
    > as I am currently under a heavy rsync load (*rolls eyes*).

    How low have you set them? Try setting them to 2 and 1 respectively.
    It cuts down fsync latencies by a significant amount in my experience.

    -Dave
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