[systemd-devel] Stateless system - Centos8 and overlayfs on root

[Preston L. Bannister]

To be clear, I am trying to introduce the minimal change possible to Centos
8 (due to outside constraints). Also past initial/lab development, the
embedded system goes on a trailer, and operates in (network) isolation for
months or years. Not so much a CI sort of setup. Did read the 0pointer
"stateless" post, before first post here.

Mucking through the systemd startup. switching root just Before local-fs
seems to cause trouble (allowing for possible other error on my part). To
my read, perhaps I need to hack systemd-remount-fs so as to preserve the
surrounding logic. Insert the overlay *after* the root volume is picked up
from /etc/fstab, and *before* the special /{proc,dev,sys} are mounted.

At least that is my present guess.


On Mon, Mar 30, 2020 at 4:18 AM Simon McVittie <smcv at collabora.com> wrote:

> On Fri, 27 Mar 2020 at 10:17:37 -0700, Preston L. Bannister wrote:
> > Updates would be accomplished by booting from the original boot menu
> entry.
> > (This is slightly complicated by the fact the target systems' computers
> do not
> > have a console - but figure I can script altering the default boot.)
>
> I think you might be better off with something based on the idea of
> building an immutable "golden image" elsewhere, and deploying it to the
> stateless machines using a filesystem-image-based "A/B" update strategy
> (like casync and Android) or a filesystem-tree-based atomic update
> strategy (like libostree, as used in Fedora Silverblue and Endless).
> Either way, the big ideas are:
>
> - build the "golden image" in CI infrastructure somewhere else
> - have enough space for two complete OSs
> - whichever one you are booted into (let's say it's "A"), overwrite the
>   other one (let's say "B") with the new OS
> - reboot into "B", but just once (have a clever bootloader)
> - if "B" fails to boot, recover by going back to "A"
>   - you hope that a later upgrade will fix the regression and overwrite
>     "B" again, this time with a better version
> - if "B" boots OK, keep using it (tell the bootloader to boot it every
>   time from now on); next time you upgrade, swap the roles of "A" and "B"
>
> Most people who do that will be using an overlay-like arrangement to
> store whatever state they want to transfer between A and B, such as /etc
> and /home - but in your case you don't want to store any state at all.
>
>     smcv
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