Objective: To safely upgrade from Debian 8 (Jessie) to Debian 9 (Stretch) two servers and to keep good records of what was done to perform the upgrade.

1. Server #1 is used just to verify and hold backups of other servers so it has no public services running.
2. Server #2 hosts a diaspora*, a Hubzilla, a GNU social, a Friendica and a GNU Mediagoblin.

The definitive source on how to achieve the objectives

There are various tutorials with very brief instructions on how to go about the upgrade, but I decided to follow  The definitive guide to upgrading from Debian Jessie (8) to Debian Stretch (9) at Debian.org to learn how to do the upgrade very carefully.

Preparation

First off: I informed the users of the free social media instances about the upcoming upgrade and the downtime to be expected.

Make sure all the software are in their latest version

# apt update && apt upgrade

Made backups of /etc, /var/lib/dpkg, /var/lib/apt/extended_states /home/username, /var/www and the output of dpkg --get-selections "*" and stored them off-site. Additionally I took a snapshot of the system disk just in case the upgrade doesn’t go well then it is possible to revert to the pre-upgrade situation.

Next checked for non-Jessie software with

$ apt-forktracer | sort

It found some items from jessie-backports but nothing that is in use.

Checked for half-installed packages with

# dpkg --audit

Nothing of interest was found. Just one dummy package.

Check for packages on hold

# dpkg --get-selections | grep 'hold$'

None were found.

Edit the /etc/apt/sources.list

Now update the /etc/apt/sources.list changing each occurrence of ‘jessie’ with ‘stretch’. I did it with sed (Stream EDitor) but it is also possible to manually edit the file with your favourite editor.

# sed -i 's/jessie/stretch/g' /etc/apt/sources.list

Start session recording for later reference

Next start session recording with (replace step with a number. When needing to reboot then restart the session recording with an incremented number)

# script -t 2>~/upgrade-stretchstep.time -a ~/upgrade-stretchstep.script

If you have used the -t switch for script you can use the scriptreplay program to replay the whole session:

# scriptreplay ~/upgrade-stretch1.time ~/upgrade-stretch1.script

The upgrade

Update the package list with the Stretch sources in place

# apt-get update

Make sure you have enough disk space for the upgrade

# apt-get -o APT::Get::Trivial-Only=true dist-upgrade

There is ample of space so proceed with minimal upgrade (upgrading only the installed software).

# apt-get upgrade

Now time to upgrade the system. This will take a while.

# apt-get dist-upgrade

Next check if you have already installed the linux-image* meta-package

# dpkg -l "linux-image*" | grep ^ii | grep -i meta

If you do not see any output, then you will either need to install a new linux-image package by hand or install a linux-image metapackage. To see a list of available linux-image metapackages, run:

# apt-cache search linux-image- | grep -i meta | grep -v transition

If unsure which linux-image metapackage you can get longer description of the linux-image in question by running

# apt-cache show linux-image-amd64

Looks good. Let’s install it.

# apt-get install linux-image-amd64

apt-get reports that there are installed packages that are no longer needed. Remove them with

# apt-get autoremove

Now it is time to reboot for the new kernel to take effect.

# reboot

Login and check the OS version

$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 9.5 (stretch)
Release:        9.5
Codename:       stretch

Verified that the services were up-and-running. Upgrade successful!

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UPDATE: It seems there are 2 PostgreSQL running: Version 9.4 (the old one from Jessie) and Version 9.6, which ships with Stretch. Going to look into removing the old one safely.