💾 Linux Disk Space Troubleshooting Guide (`df -h` at 100%)

This guide assumes your root partition (/dev/sda2 or /) is full.

1. 🔍 Identify the Biggest Space Hogs

Use the du (disk usage) command to locate the directories consuming the most space.

Change directory to root:
```
cd /
```
Run du and sort the results:
```
sudo du -sh /* | sort -h
```
This lists the sizes of all top-level directories (e.g., /var, /home, /usr) in human-readable format, sorted by size.
Drill down: If /var shows a huge size, investigate it further:
```
sudo du -sh /var/* | sort -h
```
Focus your cleanup efforts on the largest directories (often /var/log or /home).

The fastest way to free up gigabytes of space is usually by clearing system caches and logs.

The system journal can grow extremely large. Set a maximum size and remove the excess immediately.

Clean logs, keeping the last 500MB (or less, if space is critical):
```
sudo journalctl --vacuum-size=500M
```
You can substitute 500M with a lower value if needed.

Remove stored installation packages (.deb files) and unnecessary libraries/old kernels.

Clear the package cache:
```
sudo apt clean
```
Remove unused dependencies and old kernel files:
```
sudo apt autoremove --purge
```

Files that have been deleted but are still being held open by a running process (like a large log file) still consume disk space.

List open deleted files:
```
sudo lsof | grep deleted
```
Note the PID (Process ID) and the file NAME.
Truncate or restart:
- Truncate (Clear contents) a log file (safer than killing the process):
```
sudo truncate -s 0 /path/to/large/log/file
```
- Restart the associated service (e.g., sudo systemctl restart nginx).

Set a permanent size limit on your system journal to prevent future disk saturation.

Edit the configuration file:
```
sudo nano /etc/systemd/journald.conf
```
Set the maximum log usage (e.g., to 1 Gigabyte) by adding/editing this line under the [Journal] section:
```
SystemMaxUse=1G
```
Save, exit, and restart the service:
```
sudo systemctl restart systemd-journald
```

Always confirm the fix by checking the available space:

df -h

Goal: See the /dev/sda2 usage percentage drop significantly.