[OE-core] RFC: Reference updater filesystem

[Randy Witt]

On Mon, Nov 23, 2015 at 1:41 PM, Mariano Lopez <
mariano.lopez at linux.intel.com> wrote:

> There has been interest in an image based software updater in Yocto
> Project. 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.
>
> 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.
>
> 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).
>
> Configuration files can be anywhere a package decides to install them. So
having a single partition would be difficult. If you could, you would most
likely be forced to have an initramfs to make sure /etc was mounted before
init runs.


> 2. Do the backup during the update.
>   a. The pro is the configuration is directly in rootfs.
>   b. The con is If the update fail most likely the configuration would be
> lost.
>
> Why would the configuration be lost if the update fails? Couldn't it just
be stored on the thumbdrive?


> 3. Have an OverlayFS for the rootfs or the partition that have the
> configuration.
>   a. The pro is the configuration is  "directly" in rootfs.
>   b. The con is there is need to provide a custom init to guarantee the
> Overlay is mounted before the boot process.
>
> 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.
>
> Mariano
> --
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