Disk full prevents WAL file writes causing transaction failures and server shutdown

When disk space is exhausted, PostgreSQL cannot write WAL (Write-Ahead Log) files, process transactions, or allow new connections. This leads to transaction failures, potential data corruption, or complete server shutdown.

Immediately free up space: check disk usage with `df -h`, identify largest files in PostgreSQL data directory with `du -sh /var/lib/postgresql/14/main/* | sort -h`. Remove old log files older than 7 days using `find /var/lib/postgresql/14/main/log -name "*.log" -mtime +7 -delete`. Force checkpoint with `CHECKPOINT;` to recycle WAL files. Clear temporary files in `pgsql_tmp/` directory.