NervesMountManager

Monitors and reports changes to the mounted filesystems on a Nerves device.

Installation

The package can be installed by adding nerves_mount_manager to your list of dependencies in mix.exs:

def deps do
  [
    {:nerves_mount_manager, git: "https://github.com/brandonmenc/nerves_mount_manager"}
  ]
end

Example

NervesDiskMountExample is an example app that uses this module.

Todo

Lots. Not production ready.

See also

NervesDiskManager

How it works

NervesMountManager.MountWatcher polls the /proc/mounts file with a C program and when the file changes, the SystemRegistry is updated with the state of the mounted filesystems.

Each line of /proc/mounts is parsed into a keyword list. For example, if the last line is:

/dev/sda1 /mnt vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,flush,errors=remount-ro 0 0

it will parse as:

iex> SystemRegistry.match(:_) |> get_in([:state, :mounts, :mounts]) |> List.last()
[
  options: %{
    "codepage" => "437",
    "dmask" => "0022",
    "errors" => "remount-ro",
    "flush" => true,
    "fmask" => "0022",
    "iocharset" => "iso8859-1",
    "relatime" => true,
    "rw" => true,
    "shortname" => "mixed"
  },
  device: "/dev/sda1",
  mount_point: "/mnt",
  type: "vfat",
  freq: "0",
  passno: "0"
]

Notice how the flag options (like flush) are parsed.

Three lists of mounts are maintained:

iex> SystemRegistry.match(:_) |> get_in([:state, :mounts]) |> Map.keys()
[:added, :mounts, :removed]

:mounts is a list of all the currently mounted filesystems, in the above format. :added is a subset of :mounts - a list of all the filesystems that were mounted since the last time the state was updated. :removed is a list of all the filesystems that were unmounted since the last time the state was updated. The format of the entries in :added and :removed is the same as in :mounts.

Sometimes you just want to check :mounts for a specific disk partition, but you can also build more complex logic based on the :added and :removed lists.