https://stackoverflow.com/questions/23635474
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RussianThe path to the mnesia directory has to be provided to erlang VM before mnesia application is started through application configuration parameters. In Mnesia tutorial, this is done with the -Application par val VM arguments syntax.
What you call a cowboy application is probably an Erlang OTP release (built by relx as per cowboy tutorial). The solutions, quickly described in Cowboy issue #595, are as follows.
The choice between solutions really depends on style as well as some constraints. Any sufficiently complex release would use a configuration file, so it would be a good choice. vm.args seems easier to deal with. Eventually, you might need to alter the start script (for example to run several nodes from a single deployment), and include some logic to define the mnesia directory.
Provide relx with a configuration file (sys_config option)
To do so, add the following term to relx.config as documented.
{sys_config, "./config/sys.config"}.
sys.config actually is a standard Erlang configuration file, also documented. Specifying mnesia dir is done by adding a section for mnesia application. If no other configuration is required, the file would be:
[{mnesia, [{dir, "/path/to/dir"}]}].
Get relx to pass arguments to the vm (vm_args option)
The vm.args file is actually passed to the VM through -args_file option. This is a simple text file with arguments.
You would add the following term to relx.config as documented.
{vm_args, "./config/vm.args"}.
And put the following content in the vm.args file:
-mnesia dir foo
Write your own start script
relx actually creates a start script, which passes -config sys.config and args_file vm.args to the VM as required. You could modify this script or roll your own to actually pass the -mnesia dir argument to the VM.
