Backup Service, Full Disk, and an Unwanted Desktop

First distinguish start from enable. systemctl start backup.service starts it once in the current boot; systemctl enable backup.service creates the symlink under the target's .wants directory so it is pulled in at boot. If systemctl is-enabled backup.service says disabled, that alone explains the symptom. Check the unit's [Install] section with systemctl cat backup.service; if there is no WantedBy=multi-user.target (or graphical.target), enable has nothing to hook into and you must add it with a drop-in. Look for Condition...= or ExecStart pointing to a script on a filesystem that is not mounted yet, and for missing After=network-online.target or Wants=network-online.target if the backup writes to a network share. Read journalctl -u backup.service -b and the previous boot with -b -1; also check the script's PATH and permissions, since an interactive shell has a different environment. If the unit is a timer, systemctl list-timers and systemctl status backup.timer matter more than the service. A target is a grouping of units; multi-user.target is the text multi-user state, roughly the old runlevel 3, while graphical.target adds the display manager. After fixing, run systemctl daemon-reload, systemctl enable --now backup.service, and test with a reboot or systemctl isolate multi-user.target.

Triage before deleting. df -h / confirms the full filesystem; df -i / checks inode exhaustion, which looks similar but is fixed differently. Then du -xhd1 / | sort -h keeps the scan on one filesystem and shows the biggest top-level directories; drill into /var, /home, /srv, and /var/log. ls -lhS /var/log | head finds large files. journalctl --disk-usage reports the journal; journalctl --vacuum-time=14d or --vacuum-size=500M trims it safely. Check /var/cache/apt with du -sh and clean with apt clean; old kernels can be removed with apt autoremove --purge after verifying the running kernel. For an active log, do not rm it: the daemon keeps the file descriptor open and space is not reclaimed. Truncate with : > /var/log/foo.log or use logrotate with copytruncate; verify with lsof +L1 for deleted-but-open files. Samba may hold locks or temp files under /var/lib/samba; do not delete them blindly. The desktop install itself consumes space and pulls many packages; removing it may reclaim several GB. Risk: deleting a backup staging area can break the next backup, so confirm ownership and purpose before removal.

A desktop environment is not just a wallpaper. It pulls in a display manager, X or Wayland, a session manager, font and icon caches, a browser, and often a full set of GNOME services. On an 8 GB server that also runs Samba and large file copies, those services compete for RAM and I/O; the desktop's indexing and update notifiers can make an unresponsive machine worse. Security surface grows: more listening services, more packages to patch, and a graphical login that may be exposed if the display manager listens on the network. You do not need a reinstall. Identify the metapackage (ubuntu-desktop, ubuntu-desktop-minimal, or gnome-shell), remove it with apt remove and then apt autoremove --purge, and set the boot default to text: systemctl set-default multi-user.target; verify with systemctl get-default. Keep SSH and the Samba services enabled. If occasional GUI administration is needed, use X11 forwarding over SSH for a single application or a separate workstation, not a permanent desktop on the server. Add RAM only after measuring with free -h, vmstat, and iostat during a file copy; the GUI removal is reversible, whereas a reinstall is a downtime event and risks the Samba configuration.