While running Debian on a low memory machine (today probably only applicable for virtual machines) you see errors message like this?

Failed to reload daemon: Refusing to reload, not enough space available on /run/systemd.
Currently, 15.4M are free, but a safety buffer of 16.0M is enforced.

For example when running system updates:

apt-get install
apt-get update
apt-get upgrade

Then your /run tmpfs is already to small from the start. By default it is size as 10% of the available main memory, so with less then 256 MB RAM you are already close to that limit of 16.0M. Luckily, that can be changed in /usr/share/initramfs-tools/init. Search for the line

mount -t tmpfs -o “noexec,nosuid,size=${RUNSIZE:-10%},mode=0755” tmpfs /run

and change it to e.g.

mount -t tmpfs -o “noexec,nosuid,size=${RUNSIZE:-15%},mode=0755” tmpfs /run

Depending on the amount of available memory 15% might still be too small, so change accordingly. In order to apply the change during the boot, the init ramdisk has to be regenerated by calling:

update-initramfs -u

Reboot and the issue is gone.