Merge pull request #81631 from lahinson/osdocs-11570-etcd-4-5-nodes

[OSDOCS-11570]: Adding docs about 4 and 5 node architecture

Author: lahinson

modules/architecture-machine-roles.adoc

@@ -81,7 +81,7 @@ Extra controls apply to control plane machines to prevent you from deleting all
 [NOTE]
 ====
 ifndef::openshift-dedicated,openshift-rosa[]
-Exactly three control plane nodes must be used for all production deployments.
+Exactly three control plane nodes must be used for all production deployments. However, on bare metal platforms, clusters can be scaled up to five control plane nodes.
 endif::openshift-dedicated,openshift-rosa[]
 ifdef::openshift-dedicated,openshift-rosa[]
 Single availability zone clusters and multiple availability zone clusters require a minimum of three control plane nodes.

modules/etcd-node-scaling.adoc

@@ -0,0 +1,39 @@
+// Module included in the following assemblies:
+//
+// * scalability_and_performance/recommended-performance-scale-practices/recommended-etcd-practices.adoc
+
+:_mod-docs-content-type: CONCEPT
+[id="etcd-node-scaling_{context}"]
+= Node scaling for etcd
+
+In general, clusters must have 3 control plane nodes. However, if your cluster is installed on a bare metal platform, you can scale a cluster up to 5 nodes as a post-installation task. For example, to scale from 3 to 4 nodes after installation, you can add a host and install it as a control plane node. Then, the etcd Operator scales accordingly to account for the additional node.
+
+Scaling to 4-node or 5-node clusters is available only on bare metal platforms.
+
+For more information about how to scale from 3 to 4 nodes by using the Assisted Installer, see "Adding hosts with the API" and "Installing a primary control plane node on a healthy cluster".
+
+The following table shows failure tolerance for clusters of different sizes:
+
+.Failure tolerances by cluster size
+[%autowidth]
+|===
+|Cluster size |Majority |Failure tolerance
+
+|1 node
+|1
+|0
+
+|3 nodes
+|2
+|1
+
+|4 nodes
+|3
+|1
+
+|5 nodes
+|3
+|2
+|===
+
+For more information about recovering from quorum loss, see "Restoring to a previous cluster state".

post_installation_configuration/cluster-tasks.adoc

@@ -322,6 +322,11 @@ include::modules/nodes-cluster-enabling-features-cli.adoc[leveloffset=+2]
 == etcd tasks
 Back up etcd, enable or disable etcd encryption, or defragment etcd data.
 
+[NOTE]
+====
+If you deployed a bare-metal cluster, you can scale the cluster up to 5 nodes as part of your post-installation tasks. For more information, see xref:../scalability_and_performance/recommended-performance-scale-practices/recommended-etcd-practices.adoc#etcd-node-scaling_recommended-etcd-practices[Node scaling for etcd].
+====
+
 include::modules/about-etcd-encryption.adoc[leveloffset=+2]
 include::modules/etcd-encryption-types.adoc[leveloffset=+2]
 include::modules/enabling-etcd-encryption.adoc[leveloffset=+2]

scalability_and_performance/recommended-performance-scale-practices/recommended-etcd-practices.adoc

@@ -16,6 +16,14 @@ include::modules/etcd-verify-hardware.adoc[leveloffset=+1]
 * link:https://access.redhat.com/solutions/4885641[How to use `fio` to check etcd disk performance in {product-title}]
 * link:https://access.redhat.com/articles/6271341[etcd performance troubleshooting guide for {product-title}]
 
+include::modules/etcd-node-scaling.adoc[leveloffset=+1]
+
+[role="_additional-resources"]
+.Additional resources
+* link:https://docs.redhat.com/en/documentation/assisted_installer_for_openshift_container_platform/2024/html/installing_openshift_container_platform_with_the_assisted_installer/expanding-the-cluster#adding-hosts-with-the-api_expanding-the-cluster[Adding hosts with the API]
+* link:https://docs.redhat.com/en/documentation/assisted_installer_for_openshift_container_platform/2024/html/installing_openshift_container_platform_with_the_assisted_installer/expanding-the-cluster#installing-primary-control-plane-node-healthy-cluster_expanding-the-cluster[Installing a primary control plane node on a healthy cluster]
+* xref:../../backup_and_restore/control_plane_backup_and_restore/disaster_recovery/scenario-2-restoring-cluster-state.adoc#dr-restoring-cluster-state[Restoring to a previous cluster state]
+
 include::modules/move-etcd-different-disk.adoc[leveloffset=+1]
 
 [role="_additional-resources"]