How can I specify the order in which filesystems are automatically mounted?

I need a particular row in /etc/fstab to be mounted always after two other rows:

# Always mount these first
UUID=fdf38dd4-9e9d-479d-b830-2a6989958503 / ext4 noatime,discard,errors=remount-ro 0 1
UUID=2b548eb8-fa67-46ce-a998-91d587dba62f /home/.hdd ext4 errors=remount-ro 0 2

# Always mount this second
none /home/ak aufs br:/home/.ssd/ak=rw:/home/.hdd/ak=rw 0 0

The current behavior is to often reverse the order of the last two rows in an unpredictable manner. How can I specify the necessary order?

Solution 1:

One option is to postpone mounting until after /etc/fstab has completed, this gives you full control over the mount order.

Just add noauto to the mount options in fstab, and mount in rc.local.

According to the man page, mount -a mounts the entries in fstab sequentially, while adding the -F ('fork') option will mount them in parallell. It appears that the standard boot is using 'fork', presumably to reduce boot time, so I guess mounting (semi-)manually afterwards is the simplest way to regain control of the mount order.

Solution 2:

Systemd has now included a dependency option that enforces mount ordering. In your case add the fstab arguments:

x-systemd.requires-mounts-for=/,x-systemd.requires-mounts-for=/home/.hdd

The noauto option will not work if you need to mount over other mounted file systems. In my case the above worked see commit https://github.com/systemd/systemd/commit/3519d230c8bafe834b2dac26ace49fcfba139823 for more details.