systemctl daemon-reload

systemctl start servicename

# systemctl get-default
multi-user.target

systemd-analyze critical-chain
systemctl --failed

If services are defined with DefaultDependencies=no, they will be started unconditionally even in emergency mode, wchich can be wrong.

local-fs.target is configured to go to OnFailure=emergency.target. File /etc/fstab is parsed by systemd-fstab-generator to generate .mount units, which are Required.

To let mounts fail without switching to emergency mode, special attribute are introduced to fstab. http://www.freedesktop.org/software/systemd/man/systemd.mount.html

Kernel commandline:

systemd.unit=emergency

systemd.log_level=debug systemd.log_target=console console=ttyS0,38400

Enable in u-boot:

setenv bootargs ${bootargs} systemd.unit=emergency.target

Create directory “dev-mmcblk0.device.wants” and put symlink to service inside it. List of devices:

systemctl --all --full -t device

Value for unit DefaultDependencies is set to yes by default. it will add following dependencies:

• Conflicts=shutdown.target to stop service during shutdown
• Conflicts=umount.target to umount mounts durign shutdown
• Requires=basic.target and After=basic.target

• .service plain service - for running daemons
• .socket socket listeners - like inetd
• .device
• .path filesystem triggers (when file is changed)
• .mount
• .automount
• .swap
• .target
• .timer
• .snapshot
• .slice
• .scope

• Requisite - similar to require, but if not met, service will be not started (fail immediately). Usefull to perform action when removable device is inserted.
• Requisite=!local-fs.target
• BindsTo - similar to require, but also declares to stop unit when other units dissapers (aslo nice for removable devices handling)
• After - start service after other services are started completely.

• Nice=-20
• IOSchedulingClass=0
• IOSchedulingPriority=0
• StandardOutput=tty |none | journal+console