This design uses VMware vSAN to implement software-defined storage for the consolidated cluster. By using vSAN you have a high level of control on the storage subsystem.

Software-defined storage is a key technology in the SDDC. vSAN is a fully integrated hypervisor-converged storage software. vSAN creates a cluster of server hard disk drives and solid state drives, and presents a flash-optimized, highly-resilient, shared storage datastore to ESXi hosts and virtual machines. vSAN allows you to control capacity, performance, and availability on a per virtual machine basis through the use of storage policies.

Requirements and Dependencies

The software-defined storage module has the following requirements and options.

  • Minimum of 3 ESXi hosts providing storage resources to the vSAN cluster.

  • vSAN is configured as hybrid storage or all-flash storage. 

    • A vSAN hybrid storage configuration requires both magnetic devices and flash caching devices.

    • An all-flash vSAN configuration requires flash devices for both the caching and capacity tiers.  

  • Each ESXi host that provides storage resources to the cluster must meet the following requirements:

    • Minimum of one SSD. The SSD flash cache tier should be at least 10% of the size of the HDD capacity tier.

    • Minimum of two HDDs for hybrid, or two additional falsh devices for an all-flash configuration

    • RAID controller compatible with vSAN. 

    • 10 Gbps network for vSAN traffic.

    • vSphere High Availability host isolation response set to power off virtual machines. With this setting, no possibility of split-brain conditions in case of isolation or network partition exists. In a split-brain condition, the virtual machine might be powered on by two ESXi hosts by mistake.

      See design decision CSDDC-VI-VC-007 for more details.

Hybrid Mode and All-Flash Mode

vSphere offers two different vSAN modes of operation, all-flash or hybrid.

Hybrid Mode

In a hybrid storage architecture, vSAN pools server-attached capacity devices (in this case magnetic devices) and caching devices, typically SSDs or PCI-e devices to create a distributed shared datastore.

All-Flash Mode

All-flash storage uses flash-based devices (SSD or PCI-e) only as a write cache while other flash-based devices provide high endurance for capacity and data persistence.

Table 1. vSAN Mode Design Decisions

Decision ID

Design Decision

Design Justification

Design Implication


Configure vSAN in hybrid mode.

Provides a lower entry point for vSAN. If necessary, you can use an all-flash configuration.

vSAN hybrid mode does not provide the potential performance or additional capabilities such as deduplication of an all-flash configuration.

Table 2. vSAN Disk Configuration Design Decisions

Decision ID

Design Decision

Design Justification

Design Implication


Use one or more 300 GB or greater SSD and three or more traditional 1 TB or greater HDDs to create at least a single disk group.

Provides enough capacity for the management VMs and a starting point for tenant workload VMs with a minimum of 10% flash-based caching.

When using only a single disk group, you limit the amount of striping (performance) capability and increase the size of the fault domain.

Scale disk space as necessary to accommodate workload VMs. Disk requirements can be higher according to the disk size of the workload.