Home / ubuntu / Questions / 1142417
Accepted
Ali Razmdideh
Asked: 2019-05-12 07:38:04 +0800 CST

How do I set up a loop device at startup with service?

  • 772

I'm setting up a loop device for VM raw image back-file, with this command:

sudo losetup /dev/loop0 "/home/asus/VirtualBox VMs/Windows RAW/Windows 10.img"

And it works fine.

$ losetup -l
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE                                            DIO LOG-SEC
/dev/loop0         0      0         0  0 /home/asus/VirtualBox VMs/Windows RAW/Windows 10.img   0     512

But I want to make this change permanent, so I created this .sevice to run at startup:

$ cat /etc/systemd/system/loops-setup.service
[Unit]
Description=Setup loop devices
DefaultDependencies=no
Conflicts=umount.target
Before=local-fs.target
After=systemd-udev-settle.service
Required=systemd-udev-settle.service

[Service]
Type=oneshot
ExecStart=/sbin/losetup /dev/loop0 "/home/asus/VirtualBox VMs/Windows RAW/Windows 10.img"
ExecStop=/sbin/losetup -d /dev/loop0
TimeoutSec=60
RemainAfterExit=yes

[Install]
WantedBy=local-fs.target
Also=systemd-udev-settle.service

Also this service starts well manually:

sudo systemctl start loops-setup

But it failed to start at startup, and shows this error:

May 11 19:57:17 ubuntu losetup[308]: losetup: /dev/loop0: failed to set up loop device: No such file or directory

I guess its because of hierarchy of services, and I should change Before and After options but I don't know how to do this.

Thanks in advance.

mount services systemd loop-device

1 Answers

  1. Best Answer

    This issue is caused by two reasons:

    1. As @CharlesGreen mentioned systemd-udev-settle.service can not be activated:

    $ sudo systemctl enable systemd-udev-settle.service
The unit files have no installation config (WantedBy=, RequiredBy=, Also=,
Alias= settings in the [Install] section, and DefaultInstance= for template
units). This means they are not meant to be enabled using systemctl.

Possible reasons for having this kind of units are:
• A unit may be statically enabled by being symlinked from another unit's
  .wants/ or .requires/ directory.
• A unit's purpose may be to act as a helper for some other unit which has
  a requirement dependency on it.
• A unit may be started when needed via activation (socket, path, timer,
  D-Bus, udev, scripted systemctl call, ...).
• In case of template units, the unit is meant to be enabled with some
  instance name specified.

    So this service stays inactive:

    $ sudo systemctl status systemd-udev-settle.service
● systemd-udev-settle.service - udev Wait for Complete Device Initialization
   Loaded: loaded (/usr/lib/systemd/system/systemd-udev-settle.service; static; vendor preset: disabled)
   Active: inactive (dead)
     Docs: man:udev(7)
           man:systemd-udevd.service(8)

    2. The second problem is losetup command needs to access home partition to set up Windows 10.img. And this partition is not mounted at startup.

    Therefore change the service in this way, resolved problem:

    [Unit]
Description=Setup loop devices
DefaultDependencies=no
Conflicts=umount.target
Before=local-fs.target
After=systemd-udevd.service home.mount
Required=systemd-udevd.service

[Service]
Type=oneshot
ExecStart=/sbin/losetup /dev/loop0 "/home/asus/VirtualBox VMs/Windows RAW/Windows 10.img"
ExecStop=/sbin/losetup -d /dev/loop0
TimeoutSec=60
RemainAfterExit=yes

[Install]
WantedBy=local-fs.target
Also=systemd-udevd.service
    • 2