Why does not the atime option of mount always update atime of files as expected?

I needed to detect read access to certain files so I needed the file system to update the atime attribute of files the usual way.

Default mount options

In the default state the file system did not update atime as expected though I did not use the noatime option:

$ cat /etc/fstab | grep '/home '
# /home was on /dev/sda7 during installation
UUID=d7e67903-f24d-45a7-be90-6a134c9c1ae9 /home         ext4    defaults      0     2

$ mount | grep '/home '
/dev/sda7 on /home type ext4 (rw)

$ cat /etc/mtab | grep '/home '
/dev/sda7 /home ext4 rw 0 0

The atime on a file was updated only sometimes. Later I realized that the file system is mounted with the new relatime option.

$ cat /proc/mounts | grep '/home '
/dev/sda7 /home ext4 rw,relatime,user_xattr,barrier=1,data=ordered 0 0

The obvious solution does not work

So it seems that the relatime option is the default on Ubuntu 12.04. The solution seemed to be easy - explicitly state the non-default atime option:

$ sudo mount -o remount,atime /home

I was suprised that the actual mount options looked exactly the same like without the atime option.

$ mount | grep '/home '
/dev/sda7 on /home type ext4 (rw)

$ cat /etc/mtab | grep '/home '
/dev/sda7 /home ext4 rw 0 0

$ cat /proc/mounts | grep '/home '
/dev/sda7 /home ext4 rw,relatime,user_xattr,barrier=1,data=ordered 0 0

In fact the system was behaving exactly the same as without the atime option.

Questions

What is going on? Why cannot I simply use the atime option? What can I do to get the normal atime functionality?

In fact there are four five atime-related options of mount in Linux:

• noatime - atime attribute (of both files and directories) is updated only during write access.
• nodiratime - atime of directories is updated only during write access, can be combined with relatime.
• relatime / atime - atime seems to be now alias of relatime. This option causes the atime attribute to update only if the previous atime is older than mtime or ctime, or the previous atime is over 24 hours old.
• strictatime - It was formerly called atime and as such was the default option. It causes the atime attribute to update with every file access. (accessing file data, not just the metadata/attributes)
• lazytime - Linux 4.10 added a new option: only update times (atime, mtime, ctime) on the in-memory version of the file inode. The on-disk data is updated lazily (see man 8 mount for details).

Since Linux kernel 2.6.30 the default option is relatime. If you want to have the original atime functionality you must use the strictatime option. In the presented case it would be:

sudo mount -o remount,strictatime /home

Sources

• stat() - atime problems solutions
• /etc/fstab - common options to all file systems
• Once upon a time