Postgresql sudden storage increase at regular interval?
Since you have no filesystem access you won't get the answer but some guesses only, so here comes mine: if there are no bulk INSERTs, this might be some temporary space created by SELECT, for example big query with &quot;Sort Method: external merge Disk&quot;, or some TEMPORARY tables, materialized view REFRESH etc. Such query would obviously take some time, so check your query log for very long duration and EXPLAIN them if found.
The usage sliding down, however, suggests it's WAL-related. SELECT * FROM pg_stat_archiver or pg_stat_replication might show something interesting, e.g. I check for streaming lag as follows:
SELECT active, pg_wal_lsn_diff(pg_current_wal_insert_lsn(), restart_lsn) AS retained_bytes FROM pg_replication_slots
What also might happen is that the space reported itself is skewed compared to datasize. For example (this is not the case, but makes reading such graphs easier) reclaiming disk space from btrfs or after removing many files on ext3/4 filesystem might take some significant time (I've seen like 10 minutes or so on btrfs).