Dealing with read-only rootfs

[jeremy.rosen@xxxxxxxx (Jérémy Rosen)]

Hello everybody

I am trying to understand the recommanded way to deal with read-only 
rootfs...

my understanding is that (slightly simplified)
* /run must be a tmpfs
* /var must be writable, and will usually be persistant (can be a tmpfs too)
* everything else can be read-only

My problem comes with populating an empty /var. There seems to be hints 
to a systemd-provided mechanism to do that, but I can't find any 
documentation for it

Apparently, tmpfiles.d should copy the content from /usr/share/factory 
to /var when /var is empty, but I can't find what actually implements that

* is it dynamically added in /run ?
* is it just a recommanded way, and I have to implemented myself ? if 
yes, is the directory /usr/share/factory documented anywhere ?
* what qualifies as "empty /var" ? (this is mostly about being robust to 
power-loss during the copying of /var)

Any hints would be welcomed

thx
Jeremy


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