Merge branch 'eread/contrast-gitaly-cluster-and-geo' into 'master'

Contrast Gitaly Cluster and Geo

See merge request gitlab-org/gitlab!48215

doc/administration/geo/index.md

@@ -51,6 +51,11 @@ Geo provides:
 - Authentication system hooks: **Secondary** nodes receives all authentication data (like user accounts and logins) from the **primary** instance.
 - An intuitive UI: **Secondary** nodes use the same web interface your team has grown accustomed to. In addition, there are visual notifications that block write operations and make it clear that a user is on a **secondary** node.
 
+### Gitaly Cluster
+
+Geo should not be confused with [Gitaly Cluster](../gitaly/praefect.md). For more information about
+the difference between Geo and Gitaly Cluster, see [Gitaly Cluster compared to Geo](../gitaly/praefect.md#gitaly-cluster-compared-to-geo).
+
 ## How it works
 
 Your Geo instance can be used for cloning and fetching projects, in addition to reading any data. This will make working with large repositories over large distances much faster.

doc/administration/gitaly/praefect.md

@@ -47,18 +47,40 @@ The availability objectives for Gitaly clusters are:
   [Faster outage detection](https://gitlab.com/gitlab-org/gitaly/-/issues/2608)
   is planned to improve this to less than 1 second.
 
-The current version supports:
+Gitaly Cluster supports:
 
-- Eventual consistency of the secondary replicas.
-- Automatic failover from the primary to the secondary.
-- Reporting of possible data loss if replication queue is non empty.
-- Marking the newly promoted primary read only if possible data loss is
-  detected.
+- [Strong consistency](#strong-consistency) of the secondary replicas.
+- [Automatic failover](#automatic-failover-and-leader-election) from the primary to the secondary.
+- Reporting of possible data loss if replication queue is non-empty.
+- Marking repositories as [read only](#read-only-mode) if data loss is detected to prevent data inconsistencies.
 
 Follow the [HA Gitaly epic](https://gitlab.com/groups/gitlab-org/-/epics/1489)
 for improvements including
 [horizontally distributing reads](https://gitlab.com/groups/gitlab-org/-/epics/2013).
 
+## Gitaly Cluster compared to Geo
+
+Gitaly Cluster and [Geo](../geo/index.md) both provide redundancy. However the redundancy of:
+
+- Gitaly Cluster provides fault tolerance for data storage and is invisible to the user. Users are
+  not aware when Gitaly Cluster is used.
+- Geo provides [replication](../geo/index.md) and [disaster recovery](../geo/disaster_recovery/index.md) for
+  an entire instance of GitLab. Users know when they are using Geo for
+  [replication](../geo/index.md). Geo [replicates multiple datatypes](../geo/replication/datatypes.md#limitations-on-replicationverification),
+  including Git data.
+
+The following table outlines the major differences between Gitaly Cluster and Geo:
+
+| Tool           | Nodes    | Locations | Latency tolerance  | Failover                                             | Consistency                   | Provides redundancy for |
+|:---------------|:---------|:----------|:-------------------|:-----------------------------------------------------|:------------------------------|:------------------------|
+| Gitaly Cluster | Multiple | Single    | Approximately 1 ms | [Automatic](#automatic-failover-and-leader-election) | [Strong](#strong-consistency) | Data storage in Git     |
+| Geo            | Multiple | Multiple  | Up to one minute   | [Manual](../geo/disaster_recovery/index.md)          | Eventual                      | Entire GitLab instance  |
+
+For more information, see:
+
+- [Gitaly architecture](index.md#architecture).
+- Geo [use cases](../geo/index.md#use-cases) and [architecture](../geo/index.md#architecture).
+
 ## Cluster or shard
 
 Gitaly supports multiple models of scaling:
@@ -69,8 +91,8 @@ Gitaly supports multiple models of scaling:
 - Sharding using [repository storage paths](../repository_storage_paths.md), where each repository
   is stored on the assigned Gitaly node. All requests are routed to this node.
 
-| Cluster | Shard |
-|---|---|
+| Cluster                                           | Shard                                         |
+|:--------------------------------------------------|:----------------------------------------------|
 | ![Cluster example](img/cluster_example_v13_3.png) | ![Shard example](img/shard_example_v13_3.png) |
 
 Generally, Gitaly Cluster can replace sharded configurations, at the expense of additional storage
@@ -755,7 +777,7 @@ Big-IP LTM, and Citrix Net Scaler. This documentation outlines what ports
 and protocols you need configure.
 
 | LB Port | Backend Port | Protocol |
-|---------|--------------|----------|
+|:--------|:-------------|:---------|
 | 2305    | 2305         | TCP      |
 
 ### GitLab