== How you create a systemd .mount file for bind mounts

One of the types of units that systemd supports is mount units (see
'man systemd.mount'). Normally you set up all your mounts with
_/etc/fstab_ entries and you don't have to think about them, but
under some specialized circumstances you can wind up needing to
create real _.mount_ service files for some mounts.

How to specify most filesystems is pretty straightforward, but it's
not quite clear how you specify [[Linux bind mounts BindMounts]].
Since I was just [[wrestling repeatedly
https://twitter.com/thatcks/status/499258842203848704]] with this
today, here is what you need to put in a systemd _.mount_ file to
get a bind mount:

.pn prewrap on
>  [Mount]
>  What=/some/old/dir
>  Where=/the/new/dir
>  Type=none
>  Options=bind

This corresponds to the mount command '_mount --bind /some/old/dir
/the/new/dir_' and an _/etc/fstab_ line of '_/some/old/dir /some/new/dir
none bind_'. Note that the type of the mount is _none_, not _bind_
as you might expect. This works because current versions of _mount_
will accept arguments of '_-t none -o bind_' as meaning 'do a bind
mount'.

(I don't know if you can usefully add extra options to the _Options_
setting or if you'd need an actual script if you need to, eg, make
a bind mountpoint read-only. If you can do it in _/etc/fstab_ you
can probably do it here.)

A fully functioning _.mount_ unit will generally have other stuff
as well. What I've wound up using on Fedora 20 (mostly copied from
the standard _tmp.mount_) is:

>  [Unit]
>  DefaultDependencies=no
>  Conflicts=umount.target
>  Before=local-fs.target umount.target
>  
>  [Mount]
>  [[ .... whatever you need ...]]
>
>  [Install]
>  WantedBy=local-fs.target

Add additional dependencies, documentation, and so on as you need
or want them. For what it's worth, I've also had bind mount units
work without the three _[Unit]_ bits I have here.

Note that this assumes a 'local' filesystem, not a network one. If
you're dealing with a network filesystem or something depending on
one, you'll need to change bits of the targets (systemd documentation
suggests to _remote-fs.target_).