Now since we learned to mount and automount file system using systemd unit file (without using
/etc/fstab), let us continue our knowledge gathering on similar topic. Here in this article I will explain the steps to mount filesystem in certain order using systemd unit file as well as with fstab file.
With the introduction of systemd in RHEL 7 the boot process has become a lot faster because many services and processes are now started in parallel.
One of those consequences is the lack of consistent order in which filesystems are mounted. Their order for mounting is no longer guaranteed based on the entries in
/etc/fstab. Filesystems are now just another systemd unit. Because systemd defaults to parallel units execution process startup, specific target units startup order is not consistent.
In RHEL 7 and 8, systemd handles the mount order, and not the order of mount entries in
/etc/fstab. Hence, the order of entries in
/etc/fstab need not be the same in which they are mounted in RHEL 7.
Systemd Unit File Locations
systemd introduces the concept of systemd units. These units are represented by unit configuration files located in one of the directories listed in the following table.
|/usr/lib/systemd/system/||Systemd unit files distributed with installed RPM packages.|
|/run/systemd/system/||Systemd unit files created at run time. This directory takes precedence over the directory with installed service unit files.|
|/etc/systemd/system/||Systemd unit files created by systemctl enable as well as unit files added for extending a service. This directory takes precedence over the directory with runtime unit files.|
Mount filesystem in certain order using systemd
Now to demonstrate this article to mount filesystem in certain order I will create two filesystems
/dev/sdb2 -> Mounted on /first_part -> Mapped with first_part.mount unit file /dev/sdc1 -> Mounted on /second_part -> Mapped with second_part.mount unit file
We will make sure that if
first_part.mount is called before second_part.mount while the system boots so we know that we can mount filesystem in certain order.
Get UUID of the filesystem
Before we start we need the UUID of both the filesystem to configure our systemd unit file to mount filesystem in certain order.
[root@rhel-8 system]# ls -l /dev/disk/by-uuid/* lrwxrwxrwx. 1 root root 9 Sep 16 15:09 /dev/disk/by-uuid/2019-04-04-08-40-23-00 -> ../../sr0 lrwxrwxrwx. 1 root root 10 Sep 16 15:09 /dev/disk/by-uuid/2796b6a6-1080-4f7c-a902-b4438f071e6c -> ../../dm-4 lrwxrwxrwx. 1 root root 10 Sep 16 15:09 /dev/disk/by-uuid/3f46ad95-0d39-4f56-975d-2e61fc26230b -> ../../sdc1 lrwxrwxrwx. 1 root root 10 Sep 16 18:25 /dev/disk/by-uuid/716664b6-1475-4c11-9297-5920bb4f0677 -> ../../sdb2 lrwxrwxrwx. 1 root root 10 Sep 16 15:09 /dev/disk/by-uuid/abf4aa90-0b58-499a-b601-bc5f208fd2cd -> ../../sda1 lrwxrwxrwx. 1 root root 10 Sep 16 17:09 /dev/disk/by-uuid/cea0757d-6329-4bf8-abbf-03f9c313b07f -> ../../sdb1 lrwxrwxrwx. 1 root root 10 Sep 16 15:09 /dev/disk/by-uuid/e6024940-527e-4a08-ac77-0e503b219d27 -> ../../dm-3
OR you can use
[root@rhel-8 system]# blkid /dev/sdb2 /dev/sdb2: UUID="716664b6-1475-4c11-9297-5920bb4f0677" TYPE="ext4" PARTUUID="0b051d7e-02" [root@rhel-8 system]# blkid /dev/sdc1 /dev/sdc1: UUID="3f46ad95-0d39-4f56-975d-2e61fc26230b" TYPE="ext4" PARTUUID="65a7b241-01"
Sample systemd unit file
Below is my systemd unit file for first partition
[root@rhel-8 system]# pwd /usr/lib/systemd/system
[root@rhel-8 system]# cat first_part.mount [Unit] Description=Test Directory (/first_part) DefaultDependencies=no Conflicts=umount.target Before=local-fs.target umount.target After=swap.target [Mount] What=/dev/disk/by-uuid/716664b6-1475-4c11-9297-5920bb4f0677 Where=/first_part Type=ext4 Options=defaults [Install] WantedBy=multi-user.target
Below is my systemd unit file for second partition.
[root@rhel-8 system]# pwd /usr/lib/systemd/system
[root@rhel-8 system]# cat second_part.mount # This file is part of systemd. [Unit] Description=Test Directory (/second_part) DefaultDependencies=no Conflicts=umount.target Before=local-fs.target umount.target RequiresMountsFor=/first_part [Mount] What=/dev/disk/by-uuid/3f46ad95-0d39-4f56-975d-2e61fc26230b Where=/second_part Type=ext4 Options=defaults,x-systemd.requires-mounts-for=/first_part [Install] WantedBy=multi-user.target
RequiresMountsFor= Takes a space-separated list of absolute paths. Automatically adds dependencies of type Requires= and After= for all mount units required to access the specified path. x-systemd.requires-mounts-for= Configures a RequiresMountsFor= dependency between the created mount unit and other mount units. The argument must be an absolute path. This option may be specified more than once.
All other parameters are explained under this article.
/test/test1to mount only after
/testexists and is mounted.
It is important that you reload the systemd daemon after creating these systemd unit file or after every modification you do in these files
[root@rhel-8 system]# systemctl daemon-reload
Verification of the steps with examples
Now we are all done with our configuration to mount filesystem in certain order one after the other using systemd unit file. At this moment as you see none of our filesystems from this article are in mounted state.
[root@rhel-8 system]# df -h Filesystem Size Used Avail Use% Mounted on devtmpfs 900M 0 900M 0% /dev tmpfs 915M 0 915M 0% /dev/shm tmpfs 915M 8.5M 907M 1% /run tmpfs 915M 0 915M 0% /sys/fs/cgroup /dev/mapper/rhel-root 15G 2.1G 12G 16% / /dev/sda1 483M 258M 225M 54% /boot tmpfs 183M 0 183M 0% /run/user/0
Let us start our first filesystem mount service
[root@rhel-8 system]# systemctl start first_part.mount
/first_part is now mounted but
/second_part is not yet mounted
[root@rhel-8 system]# df -h | grep part /dev/sdb1 976M 2.6M 907M 1% /first_part
Similarly if we stop the first_mount service then
/first_part will be un-mounted as expected
[root@rhel-8 system]# systemctl stop first_part.mount
We get a blank output for this command which is expected
[root@rhel-8 system]# df -h | grep part
here at this stage
first_part.mount is in-active/dead
[root@rhel-8 system]# systemctl show -p ActiveState -p SubState --value second_part.mount inactive dead
Now let us start the
[root@rhel-8 system]# systemctl start second_part.mount
Here as you see we were able to mount filesystem in certain order. Once we trigerred start of
second_part.mount, it first mounted
/first_part followed by mounting
/second_part as we had planned.
Sep 17 02:57:53 rhel-8.example systemd: Mounting Test Directory (/first_part)... Sep 17 02:57:53 rhel-8.example kernel: EXT4-fs (sdb2): mounted filesystem with ordered data mode. Opts: (null) Sep 17 02:57:53 rhel-8.example systemd: Mounted Test Directory (/first_part). Sep 17 02:57:53 rhel-8.example systemd: Mounting Test Directory (second_partition)... Sep 17 02:57:53 rhel-8.example kernel: EXT4-fs (sdc1): mounted filesystem with ordered data mode. Opts: (null) Sep 17 02:57:53 rhel-8.example systemd: Mounted Test Directory (second_partition).
Verify the same using
[root@rhel-8 system]# df -h | grep part /dev/sdb2 976M 2.6M 907M 1% /first_part /dev/sdc1 976M 2.6M 907M 1% /second_part
Enable both the systemd unit service to make the changes persistent across reboots
[root@rhel-8 system]# systemctl enable first_part.mount [root@rhel-8 system]# systemctl enable second_part.mount
Mount filesystem in certain order using /etc/fstab
You can also mount filesystem in certain order using traditional
Sample /etc/fstab content
Add the filesystem and mount information depending upon your environment in
/etc/fstab as added for my usecase where I have two filesystems
[root@rhel-8 ~]# cat /etc/fstab /dev/mapper/rhel-root / ext4 defaults 1 1 UUID=abf4aa90-0b58-499a-b601-bc5f208fd2cd /boot xfs defaults 0 0 /dev/mapper/rhel-swap swap swap defaults 0 0 UUID=716664b6-1475-4c11-9297-5920bb4f0677 /first_part ext4 defaults 0 0 UUID=3f46ad95-0d39-4f56-975d-2e61fc26230b /second_part ext4 defaults,x-systemd.requires-mounts-for=/first_part 0 0
Now save and exit the file.
Since we already had created our systemd service to mount filesystem in certain order, we will disable those services so that we can verify the changes from
[root@rhel-8 system]# systemctl disable --now second_part.mount [root@rhel-8 system]# systemctl disable --now first_part.mount
This will stop the service and disable it for upcoming reboots.
Verify the changes
Next reboot the node and verify the changes
[root@rhel-8 system]# reboot login as: root email@example.com's password: Last login: Tue Sep 17 03:12:05 2019 from 10.0.2.2
We will check if our filesystem is mounted
[root@rhel-8 ~]# df -h | grep part /dev/sdb2 976M 2.6M 907M 1% /first_part /dev/sdc1 976M 2.6M 907M 1% /second_part
/etc/fstab changes are working as expected.
Since we are using
fstab to perform these changes, systemd will create a service unit file for individual
fstab entries inside
To list the systemd mount files loaded as per their order we can run below command
[root@rhel-8 ~]# ls -lt /run/systemd/generator/ total 20 -rw-r--r--. 1 root root 254 Sep 17 03:16 boot.mount -rw-r--r--. 1 root root 230 Sep 17 03:16 'dev-mapper-rhel\x2dswap.swap' -rw-r--r--. 1 root root 261 Sep 17 03:16 first_part.mount drwxr-xr-x. 2 root root 120 Sep 17 03:16 local-fs.target.requires drwxr-xr-x. 2 root root 60 Sep 17 03:16 local-fs.target.wants -rw-r--r--. 1 root root 218 Sep 17 03:16 -.mount -rw-r--r--. 1 root root 351 Sep 17 03:16 second_part.mount drwxr-xr-x. 2 root root 60 Sep 17 03:16 swap.target.requires
The systemd generated mount unit file will look similar to what we had created earlier in this article
[root@rhel-8 generator]# cat second_part.mount # Automatically generated by systemd-fstab-generator [Unit] SourcePath=/etc/fstab Documentation=man:fstab(5) man:systemd-fstab-generator(8) Before=local-fs.target RequiresMountsFor=/first_part [Mount] Where=/second_part What=/dev/disk/by-uuid/3f46ad95-0d39-4f56-975d-2e61fc26230b Type=ext4 Options=defaults,x-systemd.requires-mounts-for=/first_part
To list all the mount points which are currently loaded and active
[root@rhel-8 ~]# systemctl -t mount UNIT LOAD ACTIVE SUB DESCRIPTION -.mount loaded active mounted Root Mount boot.mount loaded active mounted /boot dev-hugepages.mount loaded active mounted Huge Pages File System dev-mqueue.mount loaded active mounted POSIX Message Queue File System first_part.mount loaded active mounted /first_part run-user-0.mount loaded active mounted /run/user/0 second_part.mount loaded active mounted /second_part sys-kernel-config.mount loaded active mounted Kernel Configuration File System sys-kernel-debug.mount loaded active mounted Kernel Debug File System LOAD = Reflects whether the unit definition was properly loaded. ACTIVE = The high-level unit activation state, i.e. generalization of SUB. SUB = The low-level unit activation state, values depend on unit type. 9 loaded units listed. Pass --all to see loaded but inactive units, too. To show all installed unit files use 'systemctl list-unit-files'.
Analyze System Manager
We can also get more information on the chain used by such mount related systemd unit file. As you see, my
second_part.mount shows the chain it will follow while bringing up the service.
[root@rhel-8 ~]# systemd-analyze critical-chain second_part.mount The time after the unit is active or started is printed after the "@" character. The time the unit takes to start is printed after the "+" character. second_part.mount +22ms └─first_part.mount @939ms +42ms └─swap.target @938ms └─dev-mapper-rhel\x2dswap.swap @819ms +95ms └─dev-mapper-rhel\x2dswap.device
For more information you can follow the man page of systemd.mount
[root@rhel-8 ~]# man systemd.mount
Lastly I hope the steps from the article to mount filesystem in certain order one after the other on CentOS/RHEL 7 and 8 Linux was helpful. So, let me know your suggestions and feedback using the comment section.