Ok we’re finally well on our way most of the migration is behind us now. Let me try to share the process we used.
First of all this is a complicated process. To catch any errors we might have missed we need a way where we can gradually migrate our devices to Mender. So that we won’t run the risk of bricking all devices in a single night. We used our own application infrastructure to be able to send a "migrate’ command to our devices.
Since we really don’t want people to pull the power during the migration, and our devices are usually on 24/7 we decided to only do the actual migrations in the dead of night (2 am local time).
We’ve put binary images of the boot partition and the system partition of our new Mender build up for download. When a device receives the migrate command it firsts starts downloading these two files and verifies their checksums. After downloading the device will wait until it’s 2 am.
For the actual migration we use two scripts on the device. The first script removes everything not needed for the migration and moves a minimal system onto a RAM disk. Then it makes all processes let go of the root file system. The last step is to kick of the second script wich will then run from the rootfs in ram.
The second script does, first reduces the old root fs to the minimum, this frees up enough space to make the new data partition behind it. Then it makes the data partition. Copies all user data over from the old root to the new data partition and the two downloded image files as well.It also sets up the needed Mender config that’s needed on the data partition.
Then it simply deletes the old root and boot partitions repartions it with a boot sys-a and sys-b partition. And used DD to write the two downloaded partition images to boot and sys-a. Then it reboots, and the device is running the new Mender managed system.
So for reference these the two migration scripts are below, I removed the company sensitive bits you’ll need to fill those places in yourself. We start the scripts with systemd.