Where does docker store volume data?

Im running a simple docker-compose.yml file containing the following directives:

services:
  postgres:
    image: postgres:12
    volumes:
      - db-data:/var/lib/postgresql/data:cached

volumes:
  db-data:

Everything works as expected and PostgreSQL has no trouble creating and finding its database files it seems. However /var/lib/postgresql does not even exist in the host filesystem. /var/lib/docker/volumes/ contains files, but where does /var/lib/postgresql/data point to if not the host filesystem?

You can inspect your volume and get the exact location on the host filesystem. Below you can find a small example of how the command output looks like, check the value of Mountpoint:

$ docker volume inspect volume_example_db-data 
[
    {
        "CreatedAt": "2021-07-30T12:34:57+03:00",
        "Driver": "local",
        "Labels": {
            "com.docker.compose.project": "volume_example",
            "com.docker.compose.version": "1.27.4",
            "com.docker.compose.volume": "db-data"
        },
        "Mountpoint": "/var/lib/docker/volumes/volume_example_db-data/_data",
        "Name": "volume_example_db-data",
        "Options": null,
        "Scope": "local"
    }
]

By default, the name of the volume gets the folder name as a prefix, in my case it was volume_example but in your case it will be something similar:

/var/lib/docker/volumes/volume_example_db-data/_data
/var/lib/docker/volumes/   name of volume    /_data

ls output:

cd /var/lib/docker/volumes/volume_example_db-data/_data
# ls
base    pg_commit_ts  pg_hba.conf    pg_logical    pg_notify    pg_serial     pg_stat      pg_subtrans  pg_twophase  pg_wal   postgresql.auto.conf  postmaster.opts
global  pg_dynshmem   pg_ident.conf  pg_multixact  pg_replslot  pg_snapshots  pg_stat_tmp  pg_tblspc    PG_VERSION   pg_xact  postgresql.conf       postmaster.pid