[yocto] Support for writable files with a read-only root file system

[Anders Darander]

* Matt Schuckmann <Matt.Schuckmann at planar.com> [141126 18:22]:
> Yes you are correct I want a small number of files into to reside in a
> writeable partition with links in the root file system to point to
> them. I guess I latched on to the volatiles mechanism because it seems
> close to meeting my needs.

The volatile-binds from meta-ro-rootfs can be used to handle exactly
this. (I.e. they doesn't have to point to tmpfs).

The 3rd alternative is the old classical approach of using a symbolic
links...

> Making entire directory hierarchies like /etc writeable seems risky,
> things that you never want changed could get changed. Furthermore it
> makes it harder to deal with setting up default versions of files. 

> Is there really no mechanism for this type of thing in OE? It seems
> like such a common thing for anybody creating a system with a ro root
> file system to need or am I missing something. 

It depends on your which version you're running. Up to daisy, you'd have
to use meta-ro-rootfs to get access to volatile-binds.

>From dizzy onward, volatile-bind was merged to oe-core, so there's no
need for an additional layer nowadays.

> If there is not such a mechanism then I will probably recreate the
> VOLATILE_BINDS mechanism that was in the MentorEmbedded layer and
> maybe call it PERSISTENT_BINDS or some such. I'm pretty new to OE so
> whatever I come up with will be pretty rough but I'd be willing to
> share my solution if there was interest. 

Use volatile-binds.

Cheers,
Anders

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