> Could someone explain more what the overlay does, is this a temporary
> area for installation or is it where additional installed material is
> kept?
To maximize the amount of stuff you can cram into the limited amount of
flash space, OpenWRT uses a highly-compressing filesystem (squashfs) for
the builtin files (mounted on /rom), and another (still compressing, but
not quite as much, because of the need to be able to modify it somewhat
efficiently) for the config files and the extra packages installed after
the fact (mounted on /overlay).
Then the two filesystems are magically merged into one (mounted on /) to
give you the best of both worlds: a highly compressed filesystem that
you can somewhat efficiently modify.
So if /overlay is small, it's because all the space is already used by
all the built-in files (the kernel, the built-in kernel modules, the
http server, the web-config tool, the ssh server, the WPA client
and server, you name it).
You can make /overlay bigger as follows:
- add flash space to your device (usually either requires good hardware
hacking skills, or otherwise buying a different machine).
- remove stuff from /rom: by building your own firmware (which is easy
with OpenWRT), you can choose exactly what gets built-in and what
doesn't, so you can throw out stuff you don't need and you can add the
stuff you do need (by adding it into the set of built-in files, you
let it benefit from the improved compression).
So I'd recommend you build your own OpenWRT firmware and that you
include ntpd and openvpn directly in your firmware image.
> From googling, it seems that it's possible to mount /overlay using
> NFS, is this practical/possible or are there any pitfalls?
You can definitely do that, and you can even mount / via NFS if
you want. To mount /overlay from NFS, you'll definitely need to build
your own firmware (since everything else than /overlay is frozen at the
time of building the firmware, so you have no other place to configure
from where to mount /overlay).
Stefan
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