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Replacing Atime With Relatime in the Kernel
eldavojohn writes "Our friend Jeremy at the Kernal Trap has dug up some interesting criticism of atime from Linus Torvalds. As Linus submitted patches to improve relatime he noted: 'I cannot over-emphasize how much of a deal it is in practice. Atime updates are by far the biggest IO performance deficiency that Linux has today. Getting rid of atime updates would give us more everyday Linux performance than all the pagecache speedups of the past 10 years, _combined_.' And later severely beat atime about the head with a pointed stick: 'It's also perhaps the most stupid Unix design idea of all times. Unix is really nice and well done, but think about this a bit: 'For every file that is read from the disk, lets do a ... write to the disk! And, for every file that is already cached and which we read from the cache ... do a write to the disk!'" Well, I guess I can expect my Linux machine to become a little bit faster!"
After I mounted my system with nodiratime and noatime, I did not 'feel' any actual speed increase. I didn't did any hard testing of course.
Disclaimer: Disregard the above post.
There is a technical reason for this.
A lot of the time, modification of a file... isn't a modification of a file. Instead, the program will delete the existing file and create a new one in its place. (There is sometimes other operations in there, like saving to a temp file, deleting the original, then renaming the temp file to the original file name.)
This means that storing the real creation time of a file means that it won't be what you expect, because the file that you think is the same file actually isn't.
(MS-DOS/Windows have something called filesystem tunneling to attempt to get around this problem. If a file is deleted and a new one created in its place (see the MSDN article linked to from there for details) within a default 15 seconds, the creation time of the old file is carried over. This technique exchanges purity and absolute correctness (not that metadata times are reliable against tamper anyway) for utility.)
I know self-replies are stupid, but I should have mentioned something else. The metadata tunneling that Windows does is much more important than it is on Unix because the filename may need to be tunneled as well. If you open a file called "somefile with a long name.txt" in an old DOS program by opening SOMEFI~1.TXT (or even in a recent program) and it does this delete/create thing, you don't want the OS to say "oh, you're making a new file called SOMEFI~1.TXT. Spiffy"; you need the original "somefile with a long name.txt" name to carry over.
/. readers (myself included) to RTFA. ;-)
The linked site explains all this, but I know the propensity of