[LEDE-DEV] Support for more than a single overlay, which is selected at boot time

[Jurgen Van Ham]

Hi,

This solution would work for the uci configuration. This device uses
daemons that keep state (e.g., in an sql database) that is not part of
the
uci files together with binary logs that are are used over boots.

In theory it is possible to use for each of this daemons two
directories and chose by mean of a symlink the active one,
but then a system to change the links for each directory at boot time,
with can also leads to inconsistencies when Murphy strikes
and we boot with a situation where not all symlinks are created to
point to the active configuration.

/etc/config -> /etc/config1
/state/sql -> /state/sql2

Regards,

Jurgen


On Thu, Mar 9, 2017 at 5:04 AM, Philip Prindeville
<philipp_subx at redfish-solutions.com> wrote:
>
>> On Mar 7, 2017, at 6:15 AM, Jurgen Van Ham <juvanham.tc at gmail.com> wrote:
>>
>> Hi all,
>>
>> I want to support multiple (currently just two) configurations on a
>> single device. One of these configurations is chosen at boot time.
>> Instead of saving and restoring the configuration each time, I came up
>> with a solution that relies on extending the overlay by splitting it
>> into two banks. There can be more than two, but I only need two called
>> 'bank_0' and 'bank_1'
>>
>> This relies on splitting the overlay file system into two (or more)
>> different banks, which are implemented as directories in the root of
>> the overlay file system. Separate file systems would require a
>> decision how to split the size of the single overlay into two overlays
>> that do not always have the same size.
>
> Um…. maybe I’m missing something but you could also have:
>
> /etc/config1/
> /etc/config2/
>
> and have /etc/config/ be a symlink to ../config1 or ../config2.
>
> Or is that too simple to possibly work?
>
> -Philip
>
>
>>
>> Until now the writable /overlay (jffs2 or other) partition is mounted
>> as the upperdir over the /rom squashfs partition as its lowerdir.
>> Instead, I consider to replace this /overlay mount by the subdirs
>> /overlay/bank_1 or /overlay_bank_2 by modifying the code of mount_root
>> from the fstools package.
>>
>> The mount_root program read the environment variable 'OVERLAY_BANK'
>> that is set by modifying two files:  /lib/preinit/80_mount_root' and
>> '/etc/init.d/done'  base-files/files. Using an environment variable
>> keeps the selection of the overlay bank in scripts. The alternative
>> would be require to integrate the selection of the bank at boot time
>> into the mount_root program, but that ties the implementation to
>> specific hardware.
>>
>> At a high level, I modified the mount_root program to use
>> '/overlay/${OVERLAY_BANK}/' instead of '/overlay/'. When /overlay does
>> not yet contain the directory ${OVERLAY_BANK}, it creates this
>> directory.
>>
>> I can imagine that this can also be useful to test new firmware and
>> make it possible to revert the upgrade by moving back to the previous
>> overlay bank.
>>
>> Before polishing my experimental implementation and sending it as a
>> patch to the mailinglist, I'd like to know whether someone else had to
>> deal with such requirement, or sees different ways to create a device
>> that can boot with one out of several configurations.
>>
>> Regards,
>>
>> Jurgen
>