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    SubjectNoatime vs relatime
    Linus Torvalds wrote:
    > What we could do is to make "relatime" updates a bit smarter.
    >
    > A bit smarter would be:
    >
    > - update atime if the old atime is <= than mtime/ctime
    >
    > Logic: things like mailers can care about whether some
    > new state has
    > been read or not. This is the current relatime.
    >
    > - update atime if the old atime is more than X seconds
    > in the past (defaulting to one day or something)
    >
    > Logic: things like tmpwatch and backup
    > software may want to remove
    > stuff that hasn't been touched in a long time, but they
    > sure don't care about "exact" atime.

    Relatime seems to be wasteful of both IO resources _and_ CPU cycles.
    Instead of performing a single IO operation (as atime does), relatime
    performs at least three IO operations and three CPU-dependent
    operations:

    1) a read IO operation to find out the old atime
    2) a read IO operation to find out the old ctime
    3) a read IO operation to find out the old mtime
    4) Comparison of "old atime is <= than mtime/ctime"
    5) Find out current time
    6) Comparison of "current time minus old atime is > X"

    People are going to wonder why all of the sudden everything is running
    so slow due to atimes being updated after a long break.

    I suggest treating atime as if it were a subsystem that is scheduled
    for an overhaul - there have been plenty of those in the past. Give
    users/distros a config option to disable atime, but default this
    option in favor of atime for a couple of kernel release cycles. Print
    a line in dmesg that states something like:

    "Warning: Atime will be disabled by default in future kernel versions,
    but you will still be able to turn it on when configuring the kernel."

    This should give a heads-up to the 0.001% of people who still use
    atime so that they know to customize this option or start using modern
    file-monitoring techniques like inotify.

    Vlad



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