Feature Brief: Cluster Types and Sizes


VMware Cloud on AWS supports several different types of clusters that accommodate use cases ranging from evaluation and experimentation to critical business applications. The ability to quickly add or remove resources in response to shifting demand makes this cloud infrastructure service powerful, flexible, and cost-efficient.

This Feature Brief explains the types of clusters and the permitted transitions so that those responsible for managing and sizing cloud resources can make architectural decisions to benefit from this dynamic capability.

Software-defined Data Center

When you create a new software-defined data center, or SDDC, you initially decide if it will consist of regular clusters that are deployed in a single AWS Availability Zone, or if increased resiliency is warranted to stretch clusters across two AZs.  This document covers single zone SDDCs, to learn about the differences, please see the Stretched Clusters Feature Brief.

An SDDC can contain up to 10 clusters by default, but you can request an increase to 20 clusters per SDDC by contacting VMware support.

The first cluster that is created runs the SDDC management components, such as the vCenter Server appliance, in addition to customer workloads. If you are anticipating a deployment with over 30 hosts, VMware recommends that you enable the advanced configuration option during deployment in order to change the SDDC management appliance setting from medium to large. If you create subsequent clusters in an SDDC, these services will also manage the additional cluster – no additional management components are deployed into the new cluster.


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Figure 1 Advanced configuration option enabling Large SDDC Appliance Size during deployment

Types of Clusters

There are three primary types of standard (single zone) clusters in an SDDC, containing one, two, or up to 16 hosts. You choose the number of hosts when first deploying the cluster, but the size can be changed after deployment – the specifics are covered in the following sections.

Single Host SDDC Starter Configuration

Although not technically a “cluster” because it is a single physical host, the single host option provides a fully functional SDDC with VMware vSphere, vSAN, and NSX-T.  Nearly all of the capabilities of a production-sized cluster are offered so that customers can evaluate and experiment with VMware Cloud on AWS for the lowest possible cost.

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Figure 2 A single host SDDC shown in the VMware Cloud on AWS console

It’s important to know that single host SDDC clusters are not patched or upgraded during their 30-day lifespan.  At the end of 30 days, these clusters are automatically terminated, and all virtual machines and data are securely removed.

Similarly, in the unlikely event of a physical host failure or other outage, a single host SDDC may be replaced with a healthy host and a default configuration. Since data is not replicated or backed up by VMware, single host SDDC failures are unrecoverable.

For these reasons, single host SDDCs are not covered by a service level agreement and are not to be used for production applications.

Customers can optionally convert a single host SDDC into a multiple host SDDC at any time during the 30-day operational period.  After conversion, the SDDC will be suitable for production workloads because data will be replicated across multiple hosts.  The multi-host cluster will be monitored, managed, and upgraded by VMware with zero downtime to applications.

2-Host SDDC Clusters

The 2-host SDDC provides fully redundant data replication and is suitable for production applications since it is covered by the VMware Cloud service level agreement (SLA). This deployment is ideal for customers that are just beginning the journey to the public cloud but do not yet require a cluster with three or more hosts.

The 2-host cluster has no time limit restrictions and receives full lifecycle management – VMware will manage, patch, and upgrade with zero downtime to applications. Customers can provision as many 2-host clusters as they wish. You can mix an SDDC with a 2-host cluster and 3+ host clusters.

Multi-Host SDDC Cluster

Production applications are most commonly deployed to an SDDC cluster that is running with at least three hosts.  Depending on resource requirements, larger clusters can have up to 16 hosts. Like the 2-host cluster, multi-host clusters are fully managed by VMware and covered by the SLA.  An SDDC can contain multiple multi-host clusters and each host in a cluster must be of the same host type.  The SDDC Host Types Feature Brief explains the different types of physical hosts available for use with VMware Cloud on AWS.

Failure Tolerance

Depending on the number of hosts in a cluster, workloads may be protected against varying degrees of failure in one or more elements of the underlying infrastructure.  This resiliency is specified via the vSAN storage policy, which uses the nomenclature “failures to tolerate,” or FTT, to indicate the extent of protection.

A single host SDDC by definition has no protection against failures – it is not covered by the SLA and is not to be used for production workloads.  The 2-host cluster employs RAID-1 to mirror data across the two separate physical hosts, so it is able to tolerate one failure, or FTT=1. Clusters with 4 or 5 hosts can use a RAID-5 storage policy which also provides FTT=1 protection.  Clusters with 6 or more hosts are able to achieve FTT=2 by selecting the RAID-6 policy.  For more details on the vSAN storage policies and failure tolerance, please see the product documentation.

Adding or Removing Hosts

Adding or removing hosts is a common operation that can be performed by cloud administrators either interactively via the web console, or through automation using the REST API or PowerCLI. The following demo shows how to manually add a host:


Alternatively, Elastic DRS (EDRS) is a policy-based service that can add or remove hosts in a production cluster as the demand for resources fluctuate.  EDRS can be tuned to prioritize performance or cost optimization and is only available with multi-host SDDC clusters.  To learn more, see the Elastic DRS Feature Brief.

There are certain restrictions pertaining to host addition and removal operations:

  • One-host clusters can be scaled up to 2 or 3 hosts before the 30-day lifespan ends
  • Two-host clusters can be expanded to 3+ hosts but cannot return to 2 hosts
  • Clusters with 3+ hosts can be expanded and contracted from 3 to 16 nodes as needed


allowed cluster sizing changes

Figure 3 Allowed transformations of cluster sizes from 1 to 16 hosts


VMware Cloud on AWS provides a range of cluster types in order to provide the proper resources and service levels to meet customer use cases. It’s easy to get started with a single host SDDC trial and then expand it to multiple hosts that offer a service level suitable for production workloads. By allowing addition and removal of hosts according to resource requirements, customers can avoid over-provisioning and over-spending on cloud infrastructure services.


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