add a page explaining the migration to rhel 9

mkdocs/docs/HPC/migration_to_rhel9.md

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+
+# Migration ro RHEL9 operation system (Tier-2)
+
+During the second half of 2024 we will migrate the oldest HPC-UGent Tier-2 clusters that are using RHEL 8 as operating system to **RHEL9 (Red Hat Enterprise Linux 9)**.
+This includes clusters skitty, joltik, doduo, accelgor, donphan and gallade (see also the [infrastructure overview](https://www.ugent.be/hpc/en/infrastructure)), as well as the Tier-2 login nodes.
+
+## Motivation
+
+Migrating to RHEL8 is done to bring all clusters in line with the most recent cluster that are already running RHEL9 (shinx).
+
+This makes the maintenance of the HPC-UGent Tier-2 infrastructure significantly easier, since we only need to take into account a single operating system version going forward.
+
+It will also bring you the latest versions in operating system software, with more features and improved security.
+
+## Impact on the login nodes
+
+As a general rule, your login node should match the operating system of the cluster you are running on. To make this more tranparent, you will be prompted when loading a cluster with a different operation system.
+
+    module swap cluster/shinx
+
+    The following have been reloaded with a version change:
+    1) cluster/doduo => cluster/shinx         3) env/software/doduo => env/software/shinx
+    2) env/slurm/doduo => env/slurm/shinx     4) env/vsc/doduo => env/vsc/shinx
+
+    We advise you to log in to a RHEL 9 login node when using the shinx cluster.
+    The banette cluster is using RHEL 9 as operating system,
+    while the login node you are logged in to is using RHEL 8.
+    To avoid problems with testing installed software or submitting jobs,
+    it is recommended to switch to a RHEL 9 login node by running 'ssh login9'.
+
+Initially there will be only one RHEL 9 login node. As needed a second one will be added.
+
+When the default cluster (doduo) migrates to RHEL 9 the corresponding login nodes will also become default (when you do `ssh vsc4xxxx@login.hpc.ugent.be`). When they are nog longer needed the old RHEL 8 login nodes will be shut down.
+
+
+## Impact on central software stack
+
+The migration to RHEL8 as operating system should not impact your workflow, everything will basically be working as it did before (incl. job submission, etc.).
+
+However, there will be impact on the availability of software that is made available via modules.
+
+**Software that was installed with an older compiler toolchain will no longer be available once the clusters have been updated to RHEL9.**
+
+This includes all software installations on top of a toolchain that is older than foss/2023a, gompi/2023a, intel/2023a, iimpi/2023a, GCC(core)/12.3.0.
+
+The `module` command will produce a clear warning when you are loading modules that are using a toolchain that will no longer be available after the cluster has been migrated to RHEL8.
+for example:
+
+    foss/2019a:
+       ___________________________________
+      /  This module will soon no longer  \
+      \  be available on this cluster!    /
+       -----------------------------------
+             \   ^__^
+              \  (xx)\_______
+                 (__)\       )\/\
+                  U  ||----w |
+                     ||     ||
+    Only modules installed with a recent toolchain will still be available
+    when this cluster has been migrated to the RHEL 9 operating system.
+    Recent toolchains include GCC(core)/12.3.0, gompi/2023a, foss/2023a,
+    iimpi/2023a, intel/2023a, and newer versions.
+
+    You should update your workflow or job script to use more recent software
+    installations, or accept that the modules you currently rely on will soon
+    no longer be available.
+
+    To request a more recent version of the software you are using,
+    please submit a software installation request via:
+
+    https://www.ugent.be/hpc/en/support/software-installation-request
+
+    The HPC-UGent Tier-2 clusters running RHEL 8 will be migrated to RHEL 9.
+
+    For more information, see https://docs.hpc.ugent.be/Linux/migration_to_rhel9/
+
+    If you have any questions, please contact hpc@ugent.be .
+
+If you require software that is currently only available with an older toolchain on the HPC-UGent Tier-2 clusters that are still running RHEL8, check via "module avail" if a more recent version is installed that you can switch to, or submit a [software installation request](https://www.ugent.be/hpc/en/support/software-installation-request) so we can provide a more recent installation of that software which you can adopt.
+
+## Planning
+
+We plan to migrate the HPC-UGent Tier-2 clusters that are still using RHEL 8 to RHEL 9 one by one, following the schedule outlined below.
+
+| ***cluster*** | ***migration start*** | ***migration completed on*** |
+| --- | ---- | --- |
+| skitty | September 2024 | |
+| joltik | October 2024 | |
+| accelgor | November 2024 | |
+| gallade | December 2024 | |
+| donphan | january 2025 | |
+| doduo (default cluster) | january 2025 | |
+| login nodes | january 2025 | |
+
+We will keep this page up to date when more specific dates have been planned.