As @ktuite noted, when you delete a form or submission in Central, all of its resources and attachments are soft-deleted (put in trash) for 30 days. During those 30 days, you can restore that data. After the 30 days, the data is purged (trash is emptied) and marked in the database as "dead".
PostgreSQL, the database used by Central, automatically vacuums up "dead" data and makes it available for re-use. When the autovacuum happens and whether or not it returns space to the operating system (as opposed to only the database) is decided by Postgres based on how it's configured and how you are using it.
Postgres doesn't always return disk space to the operating system because doing so would be an expensive operation and so there is an optimization tradeoff that it makes. True recovery (aka full vacuum or shrinking) by Postgres typically requires blocking reads/writes and twice the size of the database because it's essentially a backup and restore. This is one of the reasons why most cloud databases (e.g., RDS) can't shrink a database once it grows.
If you want to force vacuuming, you can do so:
- Run
docker exec -it central-postgres14-1 psql -U odk -W odk - Enter the default password:
odk - Run
vacuum- Reclaims space and makes it available for re-use by the database. Does not always return space to the operating system.
- Run
vacuum full- Reclaims more space, but takes much longer, locks the database so you can't use it, and requires extra disk space. Returns space to the operating system.
So in summary, if you delete data in Central, it eventually frees up space for re-use inside the database, but it may not free up space for re-use by the operating system.