Add setup instructions for setting up NFS in a typical GitLab HA cluster.

doc/administration/high_availability/nfs_host_client_setup.md

+### Configuring NFS for GitLab HA
+
+Setting up NFS for a GitLab HA setup allows all applications nodes in a cluster to share the same files and maintain data consistency. Application nodes in an HA setup act as clients while the NFS server plays host.
+
+Below are instructions for setting up an application node(client) in an HA cluster to read from and write to a central NFS server(host).
+
+#### Step 1 - Install NFS Server on Host
+
+Installing the nfs-kernel-server package allows you to share directories with the clients running the GitLab application.
+
+```
+$ apt-get update
+$ apt-get install nfs-kernel-server
+```
+
+#### Step 2 - Install NFS Common on Client
+
+The nfs-common provides NFS functionality without installing server components which we don't need running on the application nodes.
+
+```
+$ apt-get update
+$ apt-get install nfs-common
+```
+
+#### Step 3 - Export Host's Home Directory to Client
+
+In this setup we will share the home directory on the host with the client. Edit the exports file as below to share the host's home directory with the client. If you have multiple clients running GitLab you must enter the client IP addresses in line in the `/etc/exports` file.
+
+```
+#/etc/exports for one client
+/home <client-ip-address>(rw,sync,no_root_squash,no_subtree_check)
+
+#/etc/exports for three clients
+/home <client-ip-address>(rw,sync,no_root_squash,no_subtree_check) <client-2-ip-address>(rw,sync,no_root_squash,no_subtree_check) <client-3-ip-address>(rw,sync,no_root_squash,no_subtree_check)
+```
+
+Restart the NFS server after making changes to the `exports` file for the changes to take effect.
+```
+$ systemctl restart nfs-kernel-server
+```
+
+Check that traffic from the client is allowed by the firewall on the host by running the command: `sudo ufw status`. If blocked you can allow traffic from between a specific client with the command below.
+
+```
+$ sudo ufw allow from <client-ip-address> to any port nfs
+```
+
+#### Step 4 - Create Mount Points on Client
+
+Create a directroy on the client that we can mount the shared directory from the host. Please note that if your mount point directory contains any files they will be hidden once the remote shares are mounted. An empty/new directory on the client is recommended for this purpose.
+```
+$ mkdir -p /nfs/home
+```
+
+Confirm that the mount point works by mounting it on the client and checking that it is mounted with the command below:
+
+```
+$ mount <host_ip_address>:/home
+$ df -h
+```
+
+#### Step 6 - Setup Automatic Mounts on Boot
+
+Edit /etc/fstab on client as below to mount the remote shares automatically at boot. Note that GitLab requires advisory file locking, which is only supported natively in NFS version 4. NFSv3 also supports locking as long as Linux Kernel 2.6.5+ is used. We recommend using version 4 and do not specifically test NFSv3.
+
+```
+#/etc/fstab
+165.227.159.85:/home       /nfs/home      nfs4 defaults,soft,rsize=1048576,wsize=1048576,noatime,nobootwait,lookupcache=positive 0 2
+```
+Reboot the client and confirm that the mount point is mounted automatically.
+
+#### Step 7 - Setup Gitlab to Use NFS mounts
+
+When using the default Omnibus configuration you will need to share 5 data locations between all GitLab cluster nodes. No other locations should be shared. Changing the default file locations in `gitlab.rb` on the client allows you to have one main mount point and have all the required locations as subdirectories to use the NFS mount for git-data.
+
+```
+git_data_dirs({"default" => "/nfs/home/var/opt/gitlab-data/git-data"})
+user['home'] = '/nfs/home/var/opt/gitlab-data/home'
+gitlab_rails['uploads_directory'] = '/nfs/home/var/opt/gitlab-data/uploads'
+gitlab_rails['shared_path'] = '/nfs/home/var/opt/gitlab-data/shared'
+gitlab_ci['builds_directory'] = '/nfs/home/var/opt/gitlab-data/builds'
+```
+
+Save the changes in `gitlab.rb` and run `gitlab-ctl reconfigure`.