The primary PostgreSQL server's disk is at 98% and still rising

The MasterLinux school's PostgreSQL 17 database (enrolments, payments, student progress) runs on the primary ml-pg-01; the streaming replica ml-pg-02 is the standby. WAL files are archived every night to the backup server.

Last night, the disk alert on ml-pg-01 went from 85% to 98%. The /var/lib/postgresql volume (1 TB) has been gaining about 20 GB per day for several weeks, but nobody had linked this slope to a specific event. 19 GB remain. If the disk fills up, PostgreSQL will no longer be able to write WAL and will stop (PANIC): no more enrolments or payments.

You are logged in as dba on ml-pg-01, with sudo (and sudo -u postgres psql). Find out what is filling the disk and free up space without losing data.