This is part of a series of posts on backing up and restoring a backup for Matrix Synapse server. Synapse was installed using the matrix-docker-ansible deployment which while a little complicated can greatly ease management later on down the road. All the main components are in docker containers so we need to use docker to access.
As the root user run
docker exec --env-file=/matrix/postgres/env-postgres-psql matrix-postgres pg_dumpall -h matrix-postgres | gzip -c > /matrix/postgres.sql.gz
This will dump the Postgres database in /matrix/postgres.sql.gz
We can use this later to restore to a new server or keep as a backup.