tl;dr: Each and every tablespace is critical to the operation of your PostgreSQL database. If you lose one, you’ve lost the entire database.
This one can be short and sweet: If you use tablespaces in PostgreSQL, each and every one of them is a critical part of your database. If you lose one, your database is corrupted, probably irretrievably. Never, ever think that if you lose a tablespace, you’ve just lost the data in the tables and indexes on the tablespace; you’ve lost the whole database.
In a couple of cases, clients have had what they thought were clever uses of tablespaces. In each case, they could have lead to disaster:
“We’ll keep cached and recent data on a tablespace on AWS instance storage on SSDs, and the main database on EBS. If the instance storage goes away, we’ll just recreate that data from the main database.”
“We’ll keep old historical data on a SAN, and more recent data on local storage.”
In each case, there was the assumption that if the tablespace was lost, the rest of the database would be intact. This assumption is false.
Each and every tablespace is a part of your database. They are the limbs of your database. PostgreSQL is an elephant, not a lizard; it won’t regrow a limb it has to leave behind. Don’t treat any tablespace as disposable!