Today at work we were setting up a development environment for a .Net Core project using PostgreSql as it’s datastore. We decided that we set up the database server running in a container in the same way I have been running SQL Server (See recent article: Running Microsoft SQL Server in a Container on Windows 10) for the local development environment. Using the docker-compose file from this article as a basis and referring to the documentation for the postgres docker image on Docker Hub we put together a docker-compose file for PostgreSQL that looked similar to this:
version: "3"
services:
postgres:
image: "postgres"
ports:
- 5432:5432
environment:
POSTGRES_USER: "MyUser"
POSTGRES_PASSWORD: "Password!23"
POSTGRES_DB: "example"
volumes:
- C:\Docker\PostgreSql\data:/var/lib/postgresql/data
Upon running docker-compose we were greeted with the following output containing an error message:
Creating postgresql_postgres_1 ... done Attaching to postgresql_postgres_1 postgres_1 | The files belonging to this database system will be owned by user "postgres". postgres_1 | This user must also own the server process. postgres_1 | postgres_1 | The database cluster will be initialized with locale "en_US.utf8". postgres_1 | The default database encoding has accordingly been set to "UTF8". postgres_1 | The default text search configuration will be set to "english". postgres_1 | postgres_1 | Data page checksums are disabled. postgres_1 | postgres_1 | fixing permissions on existing directory /var/lib/postgresql/data ... ok postgres_1 | creating subdirectories ... ok postgres_1 | selecting dynamic shared memory implementation ... posix postgres_1 | selecting default max_connections ... 20 postgres_1 | selecting default shared_buffers ... 400kB postgres_1 | selecting default time zone ... Etc/UTC postgres_1 | creating configuration files ... ok postgres_1 | running bootstrap script ... 2020-02-25 02:38:12.326 UTC [80] FATAL: data directory "/var/lib/postgresql/data" has wrong ownership postgres_1 | 2020-02-25 02:38:12.326 UTC [80] HINT: The server must be started by the user that owns the data directory. postgres_1 | child process exited with exit code 1 postgres_1 | initdb: removing contents of data directory "/var/lib/postgresql/data" postgresql_postgres_1 exited with code 1
Notice line 19: “FATAL: data directory “/var/lib/postgresql/data” has wrong ownership”. After reading the error message we noted on line 12 it reads “fixing permissions on existing directory /var/lib/postgresql/data … ok”. Also near the top of the output on line 3 it reads “The files belonging to this database system will be owned by user “postgres”.” followed by “This user must also own the server process.”. Interesting…
So after digging around a bit we found that indeed the user “postgres” must own the files in order for the db system to read them and that the container starts up as root. It appears that line 12 is trying to fix the issue, and from what we found online it will… If the data directory is on a Linux file system. Since we are attempting to mount these files from a Windows file system, it appears that “fixing the permissions” fails. No major surprise there. So what is the work around for us poor developers working on Windows machines?
Named Volumes to the Rescue
In order to get this to work we set up a named volume. In this scenario, Docker takes care of handling the files and where they are actually stored, so we don’t readily have access to the files, but we don’t really care all that much. We just want our data to persist and not get blown away when the container gets deleted.
Here is the new (working) docker-compose file with the named volume:
version: "3"
services:
postgres:
image: "postgres"
ports:
- 5432:5432
environment:
POSTGRES_USER: "MyUser"
POSTGRES_PASSWORD: "Password!23"
POSTGRES_DB: "example"
volumes:
- psql:/var/lib/postgresql/data
volumes:
psql:
Using this approach you may want to keep an eye on the named volumes on your system and clean them up when you are no longer using them. To get a list of the volumes on your machine use the following command:
docker volumes ls
That will dump out a list of volumes on your machine that looks something like:
DRIVER VOLUME NAME local 600de9fcef37a60b93c410f9e7db6b4b7f9966faf5f6ba067cc6cb55ee851198 local ae45bfac51d4fb1813bd747cc9af10b7d141cf3affa26d79f46f405ebfa07462 local b94806ba697f79c7003481f8fd1d65599e532c0e2223800b39a2f90b087d5127 local d02adf9ab33dfa22e154d25e13c5bb383a5969c19c1dd98cfa2ac8e560d87eb4 local postgresql_psql
Notice the last entry named “postgresql_psql”? That is the one we just created above. To remove it use the following command (Note: It will not allow you to remove the volume if it is referenced by a container, running or not, so you’ll want to stop and remove the container first):
docker volume rm postgresql_psql
