What is the Mnesia directory (Mnesia.dir)?

The Mnesia directory is the filesystem location where a node stores all of its disk-based Mnesia data — the schema file, disc_copies table files, transaction logs, and backups. Its default name follows the pattern Mnesia.NodeName, but it's fully configurable.

%% in sys.config or via command line
{mnesia, [{dir, "/var/data/mnesia/mynode"}]}.

Every node running Mnesia with any disk-based storage type needs its own directory, and that directory is what makes a disc_copies table's data durable across a node restart — the schema and table data are read back from this directory when mnesia:start/0 runs again. It should live somewhere with reliable, persistent storage in a production deployment, since losing this directory on a disc_copies-only node means losing that node's durable copy of the data.

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