Systemd is the "init" system for RHEL 8 (and 7 for that matter). It replaces Upstart, the SysV "init" system used in prior releases of RHEL. Systemd is more than just a facility to bring up user space, it is a system manager that offers:
-
service parallelization
-
socket and D-Bus activation
-
on-demand starting of services
-
track services and child processes via cgroups
-
and much more
Starting on the host workstation.example.com, let’s ssh over to node1.example.com. No password should be required.
ssh node1.example.com
Verify that you are on the right host for these exercises.
cheat-systemd-checkhost.sh
You are now ready to proceed with these exercises.
Let’s collect some initial data about the boot process
systemd-analyze
Your output will look something like this
Startup finished in 3.783s (kernel) + 6.526s (initrd) + 14.723s (userspace) = 25.033s
multi-user.target reached after 10.540s in userspacesystemd-analyze blame
4.205s kdump.service
3.308s firewalld.service
3.108s dnf-makecache.service
2.991s NetworkManager-wait-online.service
2.182s dracut-initqueue.service
1.562s sssd.service
...<output truncated>...This command prints a list of all running units, ordered by the time they took to initialize. To streamline the boot process for something like a cloud image, this helps to learn the “cost” of some of the default services. Unnecessary services could potentially be removed or disabled to speed boot time.
The fundamental building block that systemd manages is called a "unit". A "unit" can describe different types of objects, but the most common type is a "service".
A "unit file" is the configuration file that describes a unit and tells systemd what dependencies exist and how to start, stop and monitor the object.
"unit files" are stored in 2 different directories. One location is reserved for the default configurations as shipped by Red Hat and the other is for customization by the local administrators.
-
Red Hat unit files: /usr/lib/systemd/system/…
-
Customizations: /etc/systemd/system/…
systemd has a concept similar to SysV init runlevels, called targets. systemd will boot to the “default target” which can be configured using the systemctl set-default command. Some common targets and their equivalent SysV runlevels are:
-
multi-user.target == runlevel 3
-
graphical.target == runlevel 5
Let’s view the current default target.
# systemctl get-default
multi-user.targetAs mentioned above, systemd has another concept called a service. A service is a type of unit which defines the traditional daemon or process. Now let us look at what services are running on the system:
# systemctl -t service
UNIT LOAD ACTIVE SUB DESCRIPTION
auditd.service loaded active running Security Auditing Service
chronyd.service loaded active running NTP client/server
crond.service loaded active running Command Scheduler
dbus.service loaded active running D-Bus System Message Bus
dracut-shutdown.service loaded active exited Restore /run/initramfs on s
firewalld.service loaded active running firewalld - dynamic firewal
...<output truncated>...Next let’s view all of the services available (ie: everything installed, running or not) on the system. The following command is similar to the older chkconfig --list as it will show both enabled and disabled services:
# systemctl list-unit-files -t service
UNIT FILE STATE
arp-ethers.service disabled
auditd.service enabled
[email protected] enabled
blk-availability.service disabled
[email protected] static
chrony-wait.service disabled
chronyd.service enabled
...<output truncated>...The state will be enabled, disabled, static, or masked. Static indicates that the unit file does not contain an "install" section used to enable the unit. In this case, the unit typically performs a one-off action or is used as a dependency of another unit and should not be run by itself.
Now that we have a good idea of what’s installed on our system, let’s get a basic lamp stack up and running.
yum install -y httpd mariadb-server mariadb systemctl enable --now httpd mariadb
|
ℹ️
|
The "enable --now" syntax was introduced in a recent release of RHEL 7 and of course now in RHEL 8. It allows for permanently enabling as well as immediately starting services in a single command. |
Created symlink /etc/systemd/system/multi-user.target.wants/httpd.service → /usr/lib/systemd/system/httpd.service.
Created symlink /etc/systemd/system/mysql.service → /usr/lib/systemd/system/mariadb.service.
Created symlink /etc/systemd/system/mysqld.service → /usr/lib/systemd/system/mariadb.service.
Created symlink /etc/systemd/system/multi-user.target.wants/mariadb.service → /usr/lib/systemd/system/mariadb.service.Now let’s check the status. You should see two separate sections in the output, one for httpd and one for mariadb.
systemctl status httpd mariadb
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
Active: active (running) since Wed 2019-05-01 21:37:54 EDT; 12s ago
Docs: man:httpd.service(8)
Main PID: 5135 (httpd)
Status: "Running, listening on: port 80"
Tasks: 213 (limit: 24007)
Memory: 26.5M
CGroup: /system.slice/httpd.service
├─5135 /usr/sbin/httpd -DFOREGROUND
├─5163 /usr/sbin/httpd -DFOREGROUND
...<output truncated>...systemd controls more than daemons or services. For this lab, we will primarily be working with service units but it’s important to know that systemd is handling the dependencies between other types: sockets, timers, mounts, swap, slices, etc. Unit files that ship with the RHEL are stored under /usr/lib/systemd/system.
Custom unit files, changes or extensions are stored under /etc/systemd/system (or /run/systemd/system for runtime changes that won’t persist).
While the defaults for unit files won’t need to be altered most of the time, there will be circumstances where changing the defaults is quite beneficial. These could include hardware or software watchdog monitoring, tunings, resource management, or many other reasons.
Create a drop-in configuration file to extend the default httpd.service unit
cheat-systemd-httpdconfig.sh
# Contents of /etc/systemd/system/httpd.service.d/50-httpd.conf
[Service]
Restart=always
OOMScoreAdjust=-1000Notify systemd of the changes.
systemctl daemon-reload systemctl status httpd
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
Drop-In: /etc/systemd/system/httpd.service.d
└─50-httpd.conf
Active: active (running) since Wed 2019-05-01 21:37:54 EDT; 11min ago
Docs: man:httpd.service(8)
Main PID: 5135 (httpd)
Status: "Running, listening on: port 80"
Tasks: 213 (limit: 24007)
Memory: 26.5M
CGroup: /system.slice/httpd.service
├─5135 /usr/sbin/httpd -DFOREGROUND
├─5163 /usr/sbin/httpd -DFOREGROUNDNotice that systemctl status displays that the unit has been extended with a drop-in file.
OOMScoreAdjust is use by the Out Of Memory killer and is an integer between -1000 (to disable OOM killing for this process) and 1000 (to make killing of this process under memory pressure very likely).
Similar to what you did in the last step, extend the mariadb.service unit with Restart=always.
This time we’ll use systemctl to create the drop-in and notify systemd of the changes.
systemctl edit mariadb
[Service]
Restart=alwaysSave and quit the editor, and view the unit
systemctl cat mariadb systemctl status mariadb
Notice that systemctl edit allows inserting the content for the drop-in and also handles the systemctl daemon-reload automatically. Also notice that systemctl cat is a quick and easy way to view the contents of a unit & and it’s drop-ins.