Add filesystems docs

Author: MoteHue

docs/filesystems.md

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+# Overview
+
+The Slurm appliance supports mounting shared filesystems using [CephFS](https://docs.ceph.com/en/latest/cephfs/) via [OpenStack Manila](https://docs.openstack.org/manila/latest/). These docs explain:
+
+- How to create the shares in OpenStack Manila.
+
+- How to configure the Slurm Appliance to mount these Manila shares.
+
+- How to switch to using Manila shares for a shared home directory.
+
+## Creating shares in OpenStack
+
+The Slurm appliance requires that the Manila shares already exist on the system. Follow the instructions below to do this.
+
+If this is the first time Manila is being used on the system, a CephFS share type will need to be created. You will need admin credentials to do this.
+
+  ```bash
+  openstack share type create cephfs-type false --extra-specs storage_protocol=CEPHFS vendor_name=Ceph
+  ```
+
+Once this exists, create a share using credentials for the Slurm project. An access rule also needs to be created, where the `access_to` argument (`openstack share access create <share> <access_type> <access_to>`) is a user that will be created in Ceph. This needs to be globally unique in Ceph, so needs to be different for each OpenStack project.
+
+  ```bash
+  openstack share create CephFS 300 --description 'Scratch dir for Slurm prod' --name slurm-production-scratch --share-type cephfs-type --wait
+  openstack share access create slurm-production-scratch cephx slurm-production
+  ```
+
+## Configuring the Slurm Appliance for Manila
+
+To mount shares onto hosts in a group, add them to the `manila` group.
+
+  ```ini
+  [manila:children]
+  login
+  compute
+  ```
+
+Set the version of Ceph which is running on the system.
+
+  ```yaml
+  os_manila_mount_ceph_version: "18.2.4"
+  ```
+
+Define the list of shares to be mounted, and the paths to mount them to. See the [stackhpc.os-manila-mount role](https://github.com/stackhpc/ansible-role-os-manila-mount) for further configuration options.
+
+  ```yaml
+  os_manila_mount_shares:
+    - share_name: slurm-production-scratch
+      mount_path: /scratch
+  ```
+
+### Shared home directory
+
+By default, the Slurm appliance configures the control node as an NFS server and exports a directory which is mounted on the other cluster nodes as `/home`. When using Manila + CephFS for the home directory instead, this will need to be disabled. To do this, set the tf var `home_volume_provisioning` to `None`. 
+
+The `basic_users_homedir_server_path` home directory will need to be updated to point to this new shared directory.
+
+  ```yaml
+  basic_users_homedir_server: "{{ groups['login'] | first }}" # if not mounting /home on control node
+  basic_users_homedir_server_path: /home
+  ```
+
+Finally, add the home directory to the list of shares (the share should be already created in OpenStack).
+
+  ```yaml
+  os_manila_mount_shares:
+    - share_name: slurm-production-scratch
+      mount_path: /scratch
+    - share_name: slurm-production-home
+      mount_path: /home
+  ```