Alternative to the filesystem overlay

[Andrew Geissler]

On Thu, Aug 15, 2019 at 12:38 PM Adriana Kobylak <anoo at linux.ibm.com> wrote:
>
> In OpenBMC we've been using an overlay to make the read-only filesystem
> (or part of it) writable. It hasn't come without its issues, like
> missing directories (openbmc/openbmc#3564) and various flavors of
> corruption (openbmc/openbmc#3578 and issues when adding files to /etc in
> the build causing files that had been previously modified on the bmc to
> become corrupted).

The overlay sounds ideal (when it works) because we don't need all of the
special code and logic you mention below. Any chance we can just fix
whatever is causing the corruption?

>
> An alternative would be to use a bind mount and manage the file merging
> by an OpenBMC app. Doing some experimentation, seems this solves the
> issues seen above. The way that it'd work would be:
> - The init script or initramfs would do an rsync from /etc to the bind
> mount destination directory in the read-write filesystem, skipping the
> contents of a list which would contain the files that have been modified
> in the BMC and should be preserved.
> - A new OpenBMC app would monitor /etc for changes via inotify for
> example, and if a file is modified, it'd add it to the "list".

We always have race condition here though? Someone updates the
BMC network info and before the app can detect it they reboot the
BMC and we lose the settings.

Any idea what other flash based distros do out there? Seems like
this would be a common problem for a lot of projects.

>
> Considerations:
> - The /etc dir mainly being a configuration destination does not get
> modified often so the app that is watching the directory would not be
> triggered often.
> - The bind mount would duplicate the contents of etc although it's not
> much. If space is a concern, some exploration could be done to have the
> bind mount be a tmpfs and have the monitor app copy the modified files
> to the read-write filesystem instead of adding them to a list.
>
> Thoughts?
>