Alternative to the filesystem overlay

[Alexander A. Filippov]

On Tue, Aug 20, 2019 at 11:00:25AM -0700, William Kennington wrote:
> Sounds like this is just a data schema update type of issue, where you
> just need a service to run at first update boot and upgrade the files
> as expected by the change.
> 
> We could probably do this with systemd services that are
> ConditionNeedsUpdate= and
> https://www.freedesktop.org/software/systemd/man/systemd-update-done.service.html
> 

Yes, it looks like a good solution. I will try it.
Thanks for the tip.

> I'd prefer to see us move to a mostly immutable filesystem with
> symlinks that map into a rwfs for specific files / directories we
> expect to mutate at runtime. I don't think we should do any smart
> types of writeback caching at the filesystem level as that will just
> lead to confusion about persistence that is hard to debug. Not to
> mention tools like rsync don't guarantee atomic snapshotting during
> the copy for file consistency. Our applications should be designed in
> such a way that they are always consistent on disk. Ex. writing text
> files should be done to a temporary, then moved over the old one. That
> way the update is atomic and we don't have partially written files.
> 

Regards,
Alexander