Udev Rule – Troubleshooting Disk Mount Issues

mountudev

I have the following content in in /etc/udev/rules.d/81-external-disk.rules:

ENV{ID_FS_UUID}=="6826692e-79f4-4423-8467-cef4d5e840c5", RUN{program}+="/bin/mount -o nofail,x-systemd.device-timeout=1 -t ext4 -U 6826692e-79f4-4423-8467-cef4d5e840c5 /backup/external"

After running:

udevadm control --reload ; udevadm trigger /dev/sdb1

It does nothing at all. However if II change the mount command for something such as /bin/touch /tmp/xyz it works.

Versions:

[root@helsinki rules.d]# rpm -qa | grep udev
libgudev1-219-19.el7_2.12.x86_64
python-pyudev-0.15-7.el7_2.1.noarch
[root@helsinki rules.d]# rpm -qa | grep systemd
systemd-libs-219-19.el7_2.12.x86_64
systemd-219-19.el7_2.12.x86_64
systemd-sysv-219-19.el7_2.12.x86_64
[root@helsinki rules.d]# cat /etc/redhat-release 
CentOS Linux release 7.2.1511 (Core)

Best Answer

This is a systemd feature. The original udev command has been replaced by systemd-udevd (see its man page). One of the differences is that it creates its own filesystem namespace, so your mount is done, but it is not visible in the principal namespace. (You can check this by doing systemctl status systemd-udevd to get the Main PID of the service, then looking through the contents of /proc/<pid>/mountinfo for your filesystem).

If you want to go back to having a shared instead of private filesystem namespace, then create a file /etc/systemd/system/systemd-udevd.service with contents

.include /usr/lib/systemd/system/systemd-udevd.service
[Service]
MountFlags=shared

or a new directory and file /etc/systemd/system/systemd-udevd.service.d/myoverride.conf with just the last 2 lines, i.e. 

[Service]
MountFlags=shared

and restart the systemd-udevd service. I haven't found the implications of doing this.

Related Question