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How to use journalctl to find the error
journalctl filters the systemd journal by unit, priority, time window and boot. Three filters in sequence turn a failure buried in thousands of lines into 3 lines. Nothing to install: the journal is already there.
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Disk full: how to find what is taking the space
du per directory, find by size and /proc for a deleted file that still holds space. Measured: 181 MB total, 120 MB in a dump.sql and 30 MB in a file removed but still open.
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How to see what is using memory on Linux
Adding up the RSS of every process overshoots the machine's total RAM and nobody can explain why. The culprit is double counting of shared pages, and PSS, not RSS, is the number that reconciles.
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