Re: Noatime vs relatime

[Bill Davidsen <davidsen@tmr.com>]

Vlad wrote:
> 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

Reading a value in memory into a CPU register is not normally considered 
an I/O operation.

> 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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--
Bill Davidsen <davidsen@tmr.com>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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