Why is VACUUM FULL locking ALL databases on the cluster? From everything I have read about VACUUM FULL, I would expect it to lock the database I'm running it on, but it renders every database in the cluster inaccessible. Is there perhaps something we might have wrong in our configuration? I do have autovacuum on, but we have a few unwieldy databases I'd like to clean up thoroughly. This is PostgreSQL v10. Thanks for any insights.
I'm considering below is the reason from PostgreSQL v10 documentation:(https://www.postgresql.org/docs/10/sql-vacuum.html)

The FULL option is not recommended for routine use, but might be useful in special cases. An example is when you have deleted or updated most of the rows in a table and would like the table to physically shrink to occupy less disk space and allow faster table scans. VACUUM FULL will usually shrink the table more than a plain VACUUM would.

VACUUM causes a substantial increase in I/O traffic, which might cause poor performance for other active sessions. Therefore, it is sometimes advisable to use the cost-based vacuum delay feature. See Section 19.4.4 for details.

https://www.postgresql.org/docs/10/runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-VACUUM-COST

In your case, it might have happened because all databases in the cluster shares common resource, if it's on the same server.