Add first draft of database doc on replication

[ci skip]

doc/administration/gitlab-geo/database.md

+# GitLab Geo database replication
+
+This document describes the minimal steps you have to take in order to
+replicate your GitLab database into another server. You may have to change
+some values according to your database setup, how big it is, etc.
+
+The GitLab primary node where the write operations happen will act as `master`,
+and the secondary ones which are read-only will act as `slaves`.
+
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+**Table of Contents**  *generated with [DocToc](https://github.com/thlorenz/doctoc)*
+
+- [GitLab Geo PostgreSQL replication](#gitlab-geo-postgresql-replication)
+    - [PostgreSQL - Configure the master server](#postgresql-configure-the-master-server)
+    - [PostgreSQL - Configure the slave server](#postgresql-configure-the-slave-server)
+    - [PostgreSQL - Initiate the replication process](#postgresql-initiate-the-replication-process)
+- [GitLab Geo MySQL replication](#gitlab-geo-mysql-replication)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+
+## GitLab Geo PostgreSQL replication
+
+The following guide assumes that:
+
+- You are using PostgreSQL 9.1 or later which includes the
+  [`pg_basebackup` tool][pgback]. As of this writing, the latest Omnibus
+  packages (8.5) have version 9.2.
+- You have a master server already set up, running PostgreSQL 9.2.x, and you
+  have a new slave server set up on the same OS and PostgreSQL version.
+- The IP of master server for our examples will be `1.2.3.4`, whereas the
+  slave's IP will be `5.6.7.8`.
+
+[pgback]: http://www.postgresql.org/docs/9.2/static/app-pgbasebackup.html
+
+### PostgreSQL - Configure the master server
+
+**For installations from source**
+
+1. Create a replication user:
+
+    ```bash
+    sudo -u postgres psql -c "CREATE USER gitlab_replicator REPLICATION LOGIN ENCRYPTED PASSWORD 'thepassword';"
+    ```
+
+1. Edit `postgresql.conf` to configure the master for streaming replication
+   (for Ubuntu that would be `/etc/postgresql/9.2/main/postgresql.conf`):
+
+    ```bash
+    listen_address = 'localhost,1.2.3.4'
+    wal_level = hot_standby
+    max_wal_senders = 5
+    checkpoint_segments = 10
+    wal_keep_segments = 10
+    hot_standby = on
+    ```
+
+    Edit these values as you see fit.
+
+1. Edit the access control on the master to allow the connection from the slave
+   in `pg_hba.conf` (for Ubuntu that would be `/etc/postgresql/9.2/main/pg_hba.conf`):
+
+    ```bash
+    host    replication     gitlab_replicator        5.6.7.8/32      md5
+    ```
+
+    Note that `5.6.7.8` is the IP of the slave.
+
+1. Restart PostgreSQL
+
+**For Omnibus installations**
+
+Edit `/etc/gitla/gitlab.rb` and add the following:
+
+```ruby
+postgresql['listen_address'] = "localhost,1.2.3.4"
+postgresql['md5_auth_cidr_addresses'] = ['5.6.7.8/32']
+postgresql['sql_replication_user'] = "gitlab_replicator"
+postgresql['wal_level'] = "hot_standby"
+postgresql['max_wal_senders'] = 10
+postgresql['wal_keep_segments'] = 10
+postgresql['hot_standby'] = "on"
+```
+
+### PostgreSQL - Configure the slave server
+
+**For installations from source**
+
+1. Edit `postgresql.conf`
+1. Restart postgres
+
+### PostgreSQL - Initiate the replication process
+
+```bash
+psql -c "select pg_start_backup('initial_backup');"
+rsync -cva --inplace --exclude=*pg_xlog* /var/lib/postgresql/9.5/main/ slave_IP_address:/var/lib/postgresql/9.5/main/
+psql -c "select pg_stop_backup();"
+```
+
+```bash
+psql -c "select pg_start_backup('initial_backup');"
+rsync -cva --inplace --exclude=*pg_xlog* /var/opt/gitlab/postgresql/data/ slave_IP_address:/var/opt/gitlab/postgresql/data/
+psql -c "select pg_stop_backup();"
+```
+
+## GitLab Geo MySQL replication
+
+TODO
+
+## Acknowledgments
+
+