Add scaling options to HA documentation

Full HA is not required in many cases. There are several options
for scaling GitLab without going full HA. This documents those
options so customers don't dive into HA when it's not really
required.

doc/administration/high_availability/README.md

-# High Availability
+# Scaling and High Availability
 
 GitLab supports several different types of clustering and high-availability.
 The solution you choose will be based on the level of scalability and
@@ -13,7 +13,7 @@ of Git, developers can still commit code locally even when GitLab is not
 available. However, some GitLab features such as the issue tracker and
 Continuous Integration are not available when GitLab is down.
 
-**Keep in mind that all Highly Available solutions come with a trade-off between
+**Keep in mind that all highly-available solutions come with a trade-off between
 cost/complexity and uptime**. The more uptime you want, the more complex the
 solution. And the more complex the solution, the more work is involved in
 setting up and maintaining it. High availability is not free and every HA
@@ -31,8 +31,9 @@ engineers, and live questions coming in from some of our customers.
 
 ## GitLab Components
 
-The following components need to be considered for an HA environment. In many
-cases components can be combined on the same nodes to reduce complexity.
+The following components need to be considered for a scaled or highly-available
+environment. In many cases components can be combined on the same nodes to reduce
+complexity.
 
 - Unicorn/Workhorse - Web-requests (UI, API, Git over HTTP)
 - Sidekiq - Asynchronous/Background jobs
@@ -41,15 +42,64 @@ cases components can be combined on the same nodes to reduce complexity.
   - [PGBouncer](pgbouncer.md) - Database pool manager
 - [Redis](redis.md) - Key/Value store (User sessions, cache, queue for Sidekiq)
   - Sentinel - Redis health check/failover manager
+- Gitaly - Provides high-level RPC access to Git repositories
 
-## Architecture Examples
+## Scalable Architecture Examples
+
+When an organization reaches a certain threshold it will be necessary to scale
+the GitLab instance. Still, true high availability may not be necessary. There
+are options for scaling GitLab instances relatively easily without incurring the
+infrastructure and maintenance costs of full high availability.
+
+GitLab recommends that an organization begin to explore scaling when they have
+around 1,000 active users. At this point increasing CPU cores and memory is
+not recommended as there are some components that may not handle increased
+load well on a single host.
+
+### Basic Scaling
+
+This is the simplest form of scaling and will work for the majority of
+cases. Backend components such as PostgreSQL, Redis and storage are offloaded
+to their own nodes while the remaining GitLab components all run on 2 or more
+application nodes.
+
+This form of scaling also works well in a cloud environment when it is more
+cost-effective to deploy several small nodes rather than a single
+larger one.
+
+- 1 PostgreSQL node
+- 1 Redis nodes
+- 2 or more GitLab application nodes (Unicorn, Workhorse, Sidekiq)
+- 1 NFS/Gitaly storage server
+
+### Full Scaling
+
+For very large installations it may be necessary to further split components
+for maximum scalability. In a fully-scaled architecture the application node
+is split into separate Sidekiq and Unicorn/Workhorse nodes. One indication that
+this architecture is required is if Sidekiq queues begin to periodically increase
+in size, indicating that there is contention or not enough resources.
+
+- 1 PostgreSQL node
+- 1 Redis nodes
+- 2 or more GitLab application nodes (Unicorn, Workhorse)
+- 2 or more Sidekiq nodes
+- 2 or more NFS/Gitaly storage servers
+
+## High Availability Architecture Examples
+
+When organizations require scaling *and* high availability the following
+architectures can be utilized. As the introduction section at the top of this
+page mentions, there is a tradeoff between cost/complexity and uptime. Be sure
+this complexity is absolutely required before taking the step into full
+high availability.
 
 For all examples below, we recommend running Consul and Redis Sentinel on
 dedicated nodes. If Consul is running on PostgreSQL nodes or Sentinel on
 Redis nodes there is a potential that high resource usage by PostgreSQL or
 Redis could prevent communication between the other Consul and Sentinel nodes.
 This may lead to the other nodes believing a failure has occurred and automated
-failover is necessary. Isolating them away from the services they monitor reduces
+failover is necessary. Isolating them from the services they monitor reduces
 the chances of split-brain.
 
 The examples below do not really address high availability of NFS. Some enterprises
@@ -77,7 +127,7 @@ the contention.
 - 2 Redis nodes
 - 3 Consul/Sentinel nodes
 - 2 or more GitLab application nodes (Unicorn, Workhorse, Sidekiq, PGBouncer)
-- 1 NFS server/appliance
+- 1 NFS/Gitaly server
 
 ![Horizontal architecture diagram](../img/high_availability/horizontal.png)
 
@@ -93,7 +143,7 @@ contention due to certain workloads.
 - 3 Consul/Sentinel nodes
 - 2 or more Sidekiq nodes
 - 2 or more Web nodes (Unicorn, Workhorse, PGBouncer)
-- 1 or more NFS servers/appliances
+- 1 or more NFS/Gitaly servers
 
 ![Hybrid architecture diagram](../img/high_availability/hybrid.png)
 
@@ -112,7 +162,7 @@ with the added complexity of many more nodes to configure, manage and monitor.
 - 2 or more Git nodes (Git over SSH/Git over HTTP)
 - 2 or more API nodes (All requests to `/api`)
 - 2 or more Web nodes (All other web requests)
-- 2 or more NFS servers/appliances
+- 2 or more NFS/Gitaly servers
 
 ![Fully Distributed architecture diagram](../img/high_availability/fully-distributed.png)