How can I add the "noatime" flag to my / filesystem without a reboot

Solution 1:

Not anymore. -o remount,noatime was an effective method of disabling atime without a reboot.

I don't know that I'd say it's a good idea, but if you intend to turn atime off, I don't see what it would be a bad idea either if you have an old enough kernel. According to the man page for mount:

MS_REMOUNT

The following mountflags can be changed: MS_RDONLY, MS_SYNCHRONOUS, MS_MANDLOCK; before kernel 2.6.16, the following could also be changed: MS_NOATIME and MS_NODIRATIME; and, additionally, before kernel 2.4.10, the following could also be changed: MS_NOSUID, MS_NODEV, MS_NOEXEC.

Solution 2:

Yes, it would work.
Or you can manually add it like:

mount -o remount,noatime /dev/sd0 /mnt

Solution 3:

Edit /etc/fstab and add the proper noatime entry to your / filesystem entry.

Remount the / filesystem with:

mount -o remount,noatime /