While configuring a Debian 11 virtual machine with 128M memory, I ran into an error when reloading systemd service configuration. The error message was as described in the title. After investigation, I found that Debian by default allocates /run space proportional to system memory. With only 128M memory, the default /run size was 16M. When executing systemctl daemon-reload, the error appeared, along with a message like:
Currently, XX.XM are free, but a safety buffer of 16.0M is enforced.
The solution is simply to increase the /run size. Modify /etc/fstab and add the following line:
none /run tmpfs defaults,size=64M 0 0
This specifies 64M space for /run. According to online sources, this should be sufficient for most cases. Then execute:
mount -o remount /run
to remount /run. After completion, use df -hT to confirm the current size of /run. Once increased, running daemon-reload works normally again.