Mount options – atime vs relatime

/  28 Comments

Have you ever asked yourself, what the difference between the mount option atime and relatime is?
In this short blog post, we tell you the main difference of these options and for what you should use relatime.

With the mount option relatime, the access time (atime) will not be written to the disc during every access. The access time (atime) will only change, if one of these two points occurs:

  • the modified time (mtime) or change time status (ctime) of a file is newer than the last access time (atime).
  • the access time (atime) is older than a defined interval (1 day by default on a RHEL system).

So the relatime mount option is a nice mix between the options atime and noatime and useful for applications, which need the access time (atime) of a file. With the relatime option, there will not be as much traffic on the disk, if a file is often accessed. For example on a webserver is the mount option relatime a good solution, because here are many read actions, but the atime will only updated once a day (with the 24 hours interval setting).

If you use a solid-state disk (SSD) and have no application which needs the access time, you should use the mount option noatime.

28 Comments

  • Dominique Barton 13.11.2015 at 09:57

    I think we need to be more specific for `relatime`, because it’s not essentially true that `relatime` is only updating the time once a day.

    `relatime` was introduced in the `2.6.20` kernel and it basically only updates the `atime` when the previous `atime` was older than the `mtime` or `ctime`. Later this behaviour was changed a bit so `relatime` will also update the `atime` if the previous `atime` was older than 24h.

    So if you don’t modify your file, then the `atime` will only updated once in 24h. But every time you modify your file (and therefor update the `mtime` timestamp), you’ll also update the `atime` timestamp because to previous one is older than the new `mtime` timestamp.

  • Dominique Barton 13.11.2015 at 10:04

    Btw. in the 4.0 kernel there’s a new feature called lazyupdate (mount option `lazytime`) which basically stores the timestamp changes in memory and flushes them later to disk, together with non-timestamp related I/O’s on the same file.

  • Mike Uchima 27.04.2019 at 15:38

    Another situation where you really should consider noatime is when you are using drive-managed SMR (shingled) drives, such as the Seagate xxxxDM004 series. Recursively sweeping through a file system with lots of small files, say to compute checksums for all the files or back the files up to another location will generate a lot of random writes due to the access time updates; and sustained random writes are problematic for drive-managed SMR. I have seen situations where this can cause I/O latencies to get so bad (multiple minutes!) that the system resets the drive, resulting in failed I/Os. On a RAID setup, this can cause drives to get kicked out of the array.

  • Jason D 05.03.2020 at 11:57

    Wow, we’ve come a long way since then, the rate technology improves is insane!

  • Isaac Wells 21.04.2020 at 12:49

    Always preferred relatime to be honest, everyones got their own reasons and preferences i guess.

  • Votre Inspection 26.10.2020 at 02:08

    So that is the real difference in time. I’ll be sure to remember this. Thanks

  • Mold Removal 10.11.2020 at 23:18

    I agree. Thank you for the article. I have an SSD being developed, and its great to know about applying mount option noatime.

  • concrete wilmington nc 12.12.2020 at 01:35

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  • concrete tuscaloosa al 22.01.2021 at 16:43

    This is very useful explanation between atime and relatime. Thank you for your wise explanation.

    • David 19.02.2021 at 00:35

      This is very helpful information. Thank you.

  • Cerritos Pro Concrete 26.02.2021 at 13:44

    This is exactly what I have been looking for. Great insight on Mount options whether to use atime or realtime. I look forward to more information. We try to upgrade tech for my concrete service business in Cerritos, we could really use them well.

  • william f lynn 03.03.2021 at 12:40

    I think this is awesome. Been looking for this explanation for awhile now.

  • New York LLC 03.03.2021 at 12:49

    This makes sense now. Thank you for explaining this.

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