Update Geo documentation for selective sync by shards

doc/gitlab-geo/configuration.md

@@ -216,19 +216,27 @@ Currently, this is what is synced:
 * Issues, merge requests, snippets, and comment attachments
 * Users, groups, and project avatars
 
-## Selective replication
-
-GitLab Geo supports selective replication, which allows admins to choose which
-groups should be replicated by secondary nodes.
-
-It is important to note that selective replication:
-
-1. Does not restrict permissions from secondary nodes.
-1. Does not hide projects metadata from secondary nodes. Since Geo currently
-relies on PostgreSQL replication, all project metadata gets replicated to
-secondary nodes, but repositories that have not been selected will be empty.
-1. Secondary nodes won't pull repositories that do not belong to the selected
-groups to be replicated.
+## Selective synchronization
+
+GitLab Geo supports selective synchronization, which allows admins to choose
+which projects should be synchronized by secondary nodes.
+
+It is important to note that selective synchronization does not:
+
+1. Restrict permissions from secondary nodes.
+1. Hide project metadata from secondary nodes.
+  * Since Geo currently relies on PostgreSQL replication, all project metadata
+    gets replicated to secondary nodes, but repositories that have not been
+    selected will be empty.
+1. Reduce the number of events generated for the Geo event log
+  * The primary generates events as long as any secondaries are present.
+    Selective synchronization restrictions are implemented on the secondaries,
+    not the primary.
+
+A subset of projects can be chosen, either by group or by storage shard. The
+former is ideal for replicating data belonging to a subset of users, while the
+latter is more suited to progressively rolling out Geo to a large GitLab
+instance.
 
 ## Upgrading Geo
 

doc/gitlab-geo/configuration_source.md

@@ -121,9 +121,9 @@ method to be enabled. Navigate to **Admin Area ➔ Settings**
 Read [Verify proper functioning of the secondary node](configuration.md#step-6-verify-proper-functioning-of-the-secondary-node).
 
 
-## Selective replication
+## Selective synchronization
 
-Read [Selective replication](configuration.md#selective-replication).
+Read [Selective synchronization](configuration.md#selective-synchronization).
 
 ## Troubleshooting
 

doc/user/admin_area/geo_nodes.md

@@ -22,12 +22,11 @@ changes on the primary!
 
 Secondaries have a number of additional settings available:
 
-| Setting                  | Description|
-|--------------------------|------------|
-| Public Key               | The SSH public key of the user that your GitLab instance runs on (unless changed, should be the user `git`). |
-| Groups to replicate      | Enable Geo selective sync for this secondary - only the selected groups will be synchronized. |
-| Repository sync capacity | Number of concurrent requests this secondary will make to the primary when backfilling repositories. |
-| File sync capacity       | Number of concurrent requests this secondary will make to the primary when backfilling files. |
+| Setting                   | Description |
+|---------------------------|-------------|
+| Selective synchronization | Enable Geo [selective sync](../../gitlab-geo/configuration.md#selective-synchronization) for this secondary. |
+| Repository sync capacity  | Number of concurrent requests this secondary will make to the primary when backfilling repositories. |
+| File sync capacity        | Number of concurrent requests this secondary will make to the primary when backfilling files. |
 
 ## Geo backfill