synapse_port_db --sqlite-database homeserver.db --postgres-config homeserver-postgres.yaml

Migrates an existing SQLite database into a new PostgreSQL database, which is how small homeservers move to a setup that scales. --sqlite-database names the .db file to copy from, and --postgres-config is a YAML file with the target database credentials. Run it with Synapse stopped. PostgreSQL is the recommended database for production, and new installs should start there directly.