update README

Author: bedroge

README.md

@@ -8,124 +8,15 @@ An alternative would be the [NixOS](https://nixos.org/).
 
 ## Installation and Configuration
 
-### Prerequisites
-
-The bootstrap process will need a clean environment with C and C++ compilers (the system version of gcc and g++ will do) as well as the `make` command. It also is very sensitive to 
-the environment, so setup a user with unset `CFFLAGS`, `CFLAGS`, `LDFLAGS`, `PKG_CONFIG_PATH` and the always harmful `LD_LIBRARY_PATH` variables.
-
-EESSI provides a Docker container for this, which can be run via Singularity.
-
-### Bootstrapping Gentoo Prefix
-Gentoo Prefix provides a bootstrap script to build the prefix, see [Gentoo Prefix Bootstrap](https://wiki.gentoo.org/wiki/Project:Prefix/Bootstrap).
-We forked [this version](https://gitweb.gentoo.org/repo/proj/prefix.git/tree/scripts/bootstrap-prefix.sh?id=e77fd01734f21ec2e9c985c28ba4eb30c1b2bc9d)
-and made some modifications. See issue [#8](https://github.com/EESSI/compatibility-layer/issues/8) for more details. 
-
-You can run our version of the bootstrap script (see `bootstrap-prefix.sh`) inside the Singularity container by executing:
-```
-singularity run docker://ghcr.io/eessi/bootstrap-prefix:centos8
-```
-
-If you want to run your own version of the bootstrap script, use:
-```
-singularity exec docker://ghcr.io/eessi/bootstrap-prefix:centos8 ./bootstrap-prefix.sh
-```
-Our version of the script allows you to pick a custom snapshot for the Portage tree. This can be done by setting `CUSTOM_SNAPSHOT_URL` to
-a URL that points to a directory, and setting `CUSTOM_SNAPSHOT_URL` to the name of a snapshot file (must be a bzip2 archive). For instance:
-```
-env CUSTOM_SNAPSHOT_URL="http://cvmfs-s0.eessi-hpc.org/snapshots" CUSTOM_SNAPSHOT_VERSION="20201209" singularity run docker://ghcr.io/eessi/bootstrap-prefix:centos8
-```
-If you want to limit the supported/installed Python version(s), you can set the environment variable `PYTHON_TARGETS` before starting the bootstrap script. By only including a Python 3 version, you can prevent Python 2 from being installed, e.g.:
-```
-env PYTHON_TARGETS="python3_8" CUSTOM_SNAPSHOT_URL="http://cvmfs-s0.eessi-hpc.org/snapshots" CUSTOM_SNAPSHOT_VERSION="20201126" singularity run docker://ghcr.io/eessi/bootstrap-prefix:centos8
-```
-
-After starting the bootstrap have a long coffee...
-
-Once the bootstrap is completed, run the script to replace some paths with symlinks into the host OS:
-
-```
-scripts/prefix-symlink-host-paths.sh
-```
-
-In order to generate the right locales for your installation, see [this item](https://wiki.gentoo.org/wiki/Project:Prefix/FAQ#Add_an_en_US.UTF-8_locale)
-of the Gentoo Prefix FAQ.
-
-### Adding the EESSI overlay and packages
-Additional packages are added in the EESSI overlay, which is based on ComputeCanada.
-You can add them manually or in an automated way by using Ansible, being Ansible the preferred way. Below you can find the two options explained.
-
-#### Ansible playbook (Option 1)
-The installation of the EESSI-specific parts can be automatically executed by running the Ansible playbook `install.yml` inside the folder `ansible/playbooks`. 
-This playbook will install the [EESSI Gentoo overlay](https://github.com/EESSI/gentoo-overlay) and a set of packages, including `Lmod` and `archspec`. See the `README` in the `ansible/playbooks` folder for more details.
-
-#### Manually (Option 2)
-First, set `EPREFIX` to the path containing your Gentoo Prefix installation, and start the prefix:
-```
-export EPREFIX=/path/to/your/prefix
-${EPREFIX}/startprefix
-```
-Ensure that the configuration directory for repositories exists:
-```
-mkdir -p ${EPREFIX}/etc/portage/repos.conf
-```
-If you used `${PYTHON_TARGETS}` during the bootstrap, be sure to set it to the same value now, e.g.:
-```
-export PYTHON_TARGETS="python3_7"
-```
-
-Next, configure and sync the overlay:
-```
-emerge eselect-repository
-eselect repository add eessi git https://github.com/EESSI/gentoo-overlay.git
-emerge --sync eessi
-```
-
-After synchronizing the overlay, add the EESSI package set(s) that you would like to install, e.g. for set `2020.08`:
-```
-mkdir ${EPREFIX}/etc/portage/sets/
-ln -s  ${EPREFIX}/var/db/repos/eessi/etc/portage/sets/2020.08 ${EPREFIX}/etc/portage/sets/
-```
-
-Finally, install the package set(s) defined at `${EPREFIX}/etc/portage/sets/`, e.g.:
-```
-emerge @2020.08
-```
-
-### Updating the Prefix
-#### Packages
-Updating packages can be as easy as
-```
-emerge --sync
-emerge
-```
-If you run into problems, usually a newer ebuild is not suited to build in a prefix environment.
-Try to mask latest versions:
-
-Create a mask file if not existing and mask newer versions from thin provisioning tools greater or equal to 0.7.6:
-```
-echo ">=sys-block/thin-provisioning-tools-0.7.6" >> ${EPREFIX}/etc/portage/package.mask
-```
-
-#### Portage
-Updating Portage requires the kernel source which corresponds to your running kernel on the host. Emerge will detect it in `/usr/src/linux`.
-
-Check your running kernel version with:
-```
-cat /proc/version
-Linux version 4.20.0-1.el7.elrepo.x86_64 (mockbuild@Build64R7) 
-```
-
-On a Centos 7 host kernel sources are installed in `/usr/src/kernels`. Link `/usr/src/linux` to the appropiate kernel source after installation. Example for an `elrepo` kernel:
-```
-rpm -ivh kernel-ml-devel-4.20.0-1.el7.elrepo.x86_64.rpm
-cd /usr/src ; ln -s kernels/4.20.0-1.el7.elrepo.x86_64 linux
-```
-
- When ready update Portage from the Prefix environment:
-```
-startprefix
-emerge --oneshot sys-apps/portage
-```
+The installation of the compatibility layer is implemented with an Ansible playbook and Ansible role,
+which you can find in the `ansible` subdirectory. It will do the Gentoo Prefix bootstrap (which usually takes several hours to complete!),
+add a lot of EESSI configurations and customizations, install a bunch of packages that we require for EESSI, and finally it will
+run a ReFrame test suite to check the installation.
+
+To make the installation even easier, we provide a script `install_compatibility_layer.sh` that can be used on basically any host that has Apptainer installed,
+without requiring special privileges.
+The script will execute the Ansible playbook inside an Apptainer build container, ensuring that all dependencies (including Ansible itself) are available.
+In order to be able to write to `/cvmfs`, the container will bind mount a directory from the host as `/cvmfs` inside the container.
 
 # License