[OE-core] RFC: Reference updater filesystem

[Anders Darander]

* Mariano Lopez <mariano.lopez at linux.intel.com> [151123 22:41]:

> There has been interest in an image based software updater in Yocto Project.

Ok. Sure, it might be nice with something that can be shared, instead
off everyone's building our own solutions.

> The proposed solution for a image based updater is to use Stefano Babic's
> software updater (http://sbabic.github.io/swupdate). This software do a
> binary copy, so it is needed to have at least two partitions, these
> partitions would be the rootfs and the maintenance partition. The rootfs
> it's the main partition used to boot during the normal device operation, on
> the other hand, the maintenance will be used to update the main partition.

I haven't checked the swupdate tool, though I'd suspect that it also
supports the alternating rootfs use case? (I.e. run system1 update
system2; reboot to system2. Next update is system1). This is a rather
common setup, not least when you need a remote upgrade facility.

Would your proposed inclusion to the Yocto Project support that case
too?

> To update the system, the user has to connect to device and boot in the
> maintenance partition; once in the maintenance partition the software
> updater will copy the new image in the rootfs partition. A final reboot into
> the rootfs it is necessary to complete the upgrade.

Like said above, not all system can be reached manually (at least not in
cost efficient way). Sure, the mainenance partition scheme can be made
to work anyway...

> As mentioned before the the software will copy an image to the partition, so
> everything in that partition will be wiped out, including custom
> configurations. To avoid the loss of configuration I explore three different
> solutions:
> 1. Use a separate partition for the configuration.
>   a. The pro of this method is the partition is not touched during the
> update.
>   b. The con of this method is the configuration is not directly in rootfs
> (example: /etc).

I'd vote for that as well. Though, I only keep the re-writable
configurations here. The one that are constant between all systems are
shipped in /etc in the read-only-rootfs.

> With the above information I'm proposing to use a separate partition for the
> configuration; this is because is more reliable and doesn't require big
> changes in the current architecture.

> So, the idea is to have 4 partitions in the media:
> 1. boot. This is the usual boot partition
> 2. data. This will hold the configuration files. Not modified by updates.
> 3. maintenance. This partition will be used to update rootfs.
> 4. rootfs. Partition used for normal operation.

How flexible to you intend to make this system? Allow everything that
swupdate supports? Or a specific subset?

Cheers,
Anders

-- 
Anders Darander, Senior System Architect
ChargeStorm AB / eStorm AB