Question

Is there a standard place to put the mnesia database within erlang? At the moment I am putting it in the priv directory.

Answer 1

By default, Mnesia will create the schema in a subdirectory called Mnesia.<node name> of the current directory of the emulator process.
If that’s not what you want, all you need to do is set Mnesia’s dir application variable with something like

application:set_env(mnesia, dir, "/path/to/db").

As for where to place the database: that depends on your deployment scenario. Since application
variables can also be set using release config files or command line arguments, you can delay that
decision until you ship (or install on your own servers). For use in production, a standard directory like /var/lib/<your application>/mnesia-<node name>(on unix) should do.

For playing around, i’d recommend using a dedicated directory under the code root (NOT the priv directory) and setting that location within your application’s startup section.
In my pet projects, i often use code such as

Root = filename:absname_join(filename:dirname(?FILE), ".."),
application:set_env(mnesia, dir, filename:join(Root, "db")).

for exactly that purpose.