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 0The 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 0The 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 /homeI 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 0In 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?
1 Answer
In fact there are four five atime-related options of mount in Linux:
noatime-atimeattribute (of both files and directories) is updated only during write access.nodiratime-atimeof directories is updated only during write access, can be combined with relatime.relatime/atime-atimeseems to be now alias ofrelatime. This option causes theatimeattribute to update only if the previousatimeis older thanmtimeorctime, or the previousatimeis over 24 hours old.strictatime- It was formerly calledatimeand as such was the default option. It causes theatimeattribute 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 (seeman 8 mountfor 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
