Installation-instructions.md

+# Install packages on debian host system
+```bash
+apt install acl dnf rpm git systemd-container python3-venv
+```
+
+# Clone git and checkout config files
+```bash
+git clone https://gitlab.physik.uni-muenchen.de/Nikolai.Hartmann/xcache-nspawn-lrz.git
+# as root in repository dir
+cd xcache-nspawn-lrz
+# optional, in order to be able to push into the dir if no modifications
+git config receive.denyCurrentBranch updateInstead
+# remove everything, except .git
+rm -r *
+# checkout files
+git reset --hard
+# check tracked files
+git ls-files /
+# remove README
+rm /README.md
+git update-index --assume-unchanged /README.md
+```
+
+# Build image
+```bash
+cd /var/local/mkosi
+# if not existing yet
+mkdir mkosi.cache
+# seems mkosi-10 from backports is not working
+# most recent version from github works
+# The following commands will install mkosi from the latest github version
+# into a virtual environment inside the directory .venv
+python3 -m venv .venv
+source .venv/bin/activate
+python3 -m pip install git+https://github.com/systemd/mkosi.git
+# generate image (mkosi command available after source .venv/bin/activate)
+mkosi -i -t directory -o /var/lib/machines/xcache-$(date +%Y-%m-%d-%H%M)
+```
+
+# Create xrootd user in host system
+We will map the xrootd user id inside the container (typically 999) to one shifted by 65536 * 10 on the host system. This is an arbitrary choice and one should check this range of user ids is not used by anything else.
+```bash
+# for some reason that command might add a carriage return (therefore the sed) ...
+XRD_UID_INSIDE=$(systemd-nspawn -D /var/lib/machines/<img-name> id -u xrootd | sed 's/\r//')
+let "XRD_UID_HOST = $XRD_UID_INSIDE + 65536 * 10"
+useradd --no-create-home --system -u $XRD_UID_HOST xrootd
+```
+
+# Directories for bind mounts
+Create directories for bind mounts and make them owned by xrootd where nescessary
+```bash
+mkdir -p /var/spool/xrootd /var/run/xrootd /srv/data/xrootd/namespace /srv/data/xrootd/xrdcinfos /srv/data/xrootd/data
+chown xrootd /var/spool/xrootd /var/run/xrootd /srv/data/xrootd/namespace /srv/data/xrootd/xrdcinfos /srv/data/xrootd/data
+
+# if not existing
+mkdir -p /etc/grid-security
+# if running with authentication and need host key
+setfacl -m "u:xrootd:r" /etc/grid-security/hostkey.pem
+```
+
+# Start and enable xcache service running in container
+The running image has to be linked to `/var/lib/machines/xcache` in order to pick up the configuration in `/etc/systemd/nspawn/xcache.nspawn`.
+```
+ln -s /var/lib/machines/{<img-name>,xcache}
+machinectl start xcache
+machinectl enable xcache
+```
\ No newline at end of file