Merge pull request #277091 from fengzhou-msft/patch-1

Update Availability zone support section

Author: ttorble

articles/reliability/reliability-hdinsight-on-aks.md

@@ -32,13 +32,45 @@ This article describes reliability support in [Azure HDInsight on Azure Kubernet
 
 [!INCLUDE [next step](includes/reliability-availability-zone-description-include.md)]
 
-Currently, Azure HDInsight on AKS doesn't support availability zone in its service offerings.
+Azure HDInsight on AKS supports availability zone by leveraging Azure Kubernetes Service's ability to create zone redundant node pools. You can select which availability zones to deploy the cluster pool and cluster during their creation. Once the cluster pool or cluster is created, you can't change the availability zones.
+
+### Prerequisites
+
+- Availability zones are only supported for cluster pool version >= `1.2` and cluster version >= `1.2.1`.
+- Azure HDInsight on AKS only has one default SKU and it supports AZ as long as the Azure region has AZ support.
+
+  Below regions don't support AZ:
+
+  | Americas         | Europe               | Middle East   | Africa             | Asia Pacific   |
+  |------------------|----------------------|---------------|--------------------|----------------|
+  | West US          | Germany North        |               |                    |                |
+- Some VM SKUs may not support all availability zones in a region. If you select those SKUs, HDInsight on AKS cluster pools or clusters don't support corresponding availability zones either.
+
+### SLA improvements
+
+There are no increased SLAs for Azure HDInsight on AKS clusters with availability zones enabled.
+
+#### Create a resource with availability zone enabled
+
+- Cluster Pools
+  You can select one or more availability zones during cluster pool creation after you select the region.
+
+- Clusters
+  You can select one or more availability zones during cluster creation.
+
+### Fault tolerance
+
+To prepare for availability zone failure, it's recommended to over-provision capacity of service to ensure that your cluster can tolerate the loss of capacity from one availability zone down and continue to function without degraded performance during zone-wide outages. For instance, if you enable 3 availability zones, your cluster should tolerate 1/3 of the nodes down (round up to the nearest integer).
+
+### Zone down experience
+
+Azure HDInsight on AKS service is zone redundant. During a zone-wide outage, the customer should expect degradation of performance due to capacity drop. Customers can still create new cluster pools and clusters in the availability zones that are not impacted. Existing clusters can function with reduced capacity. Individual open source workloads recommendations and best practices are provided on the documentation. 
 
 ## Disaster recovery and business continuity
 
 [!INCLUDE [introduction to disaster recovery](includes/reliability-disaster-recovery-description-include.md)]
 
-Currently, Azure HDInsight on AKS CP(Control Plane) service and databases are deployed across regions of Azure. Among these regions, the Azure HDInsight on AKS instances and database instances are isolated. When an outage at region level occurs, one region is down. All the resources in this region, including the RP(Resource Provider) of Azure HDInsight on AKS CP, database of Azure HDInsight on AKS CP and all customer clusters in this region. In this case, we can only wait for the regional outage to end. When the outage is recovered, the Azure HDInsight on AKS service is back and all customer clusters are back, too. It's possible that there may be some problems due to data inconsistency after the outage and needs a manual fix.
+Azure HDInsight on AKS control plane service and databases are deployed across regions of Azure. Among these regions, the Azure HDInsight on AKS instances and database instances are isolated. When an outage at region level occurs, one region is down. All the resources in this region, including the RP (Resource Provider) of Azure HDInsight on AKS control plane, database of Azure HDInsight on AKS control plane and all customer clusters in this region. In this case, we can only wait for the regional outage to end. When the zonal outage is fully recovered, Azure HDInsight on AKS service is back and all customer clusters are back to normalcy. It's possible you may encounter some problems due to data inconsistency after the outage and may need a manual fix based on your application workloads.
 
 ### Multi-region disaster recovery
 
@@ -52,13 +84,13 @@ Azure HDInsight on AKS currently doesn't support cross-region failover. Improvin
 
 ### Single-region disaster recovery
 
-Currently, Azure HDInsight on AKS only has one standard service offering and clusters are created in a single-region geography. Customers are responsible for diaster recovery.
+Currently, Azure HDInsight on AKS only has one standard service offering and clusters are created in a single-region geography. Customers are responsible for diaster recovery settings based on the application requirements.
 
 ### Capacity and proactive disaster recovery resiliency
 
-Azure HDInsight on AKS and its customers operate under the Shared responsibility model, which means that the customer must address DR for the service they deploy and control. To ensure that recovery is proactive, customers should always predeploy secondaries because there's no guarantee of capacity at time of impact for those who haven't preallocated.
+Azure HDInsight on AKS and its customers operate under the Shared responsibility model, which means that the customer must address disaster recovery requirements for the service they deploy and control. To ensure that recovery is proactive, customers should always predeploy secondaries because there's no guarantee of capacity at time of impact for those who haven't preallocated.
 
-Unlike the original version of HDInsight, the Virtual Machines used in HDInsight on AKS clusters require the same Quota as Azure VMs. For more information, see [Capacity planning](../hdinsight-aks/virtual-machine-recommendation-capacity-planning.md#capacity-planning).
+Unlike HDInsight, the Virtual Machines used in HDInsight on AKS clusters require the same Quota as Azure VMs. For more information, see [Capacity planning](../hdinsight-aks/virtual-machine-recommendation-capacity-planning.md#capacity-planning).
 
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