Software-defined storage is a key technology in the SDDC. This design uses vSAN to implement software-defined storage.

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 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 4 ESXi hosts providing storage resources to the ROBO vSAN cluster is required for guarantee vSAN redundancy during maintenance operations.

  • 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 vSphere 6.0 or later.  

  • 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 HHDs.

    • RAID controller compatible with vSAN. 

    • 10 Gbps network for vSAN traffic with Multicast enabled.

    • vSphere High Availability 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 hosts by mistake. For more information, see design decision ROBO-VI-VC-007

Table 1. vSAN Physical Storage Design Decision 

Decision ID

Design Decision

Design Justification

Design Implication


Use one 300 GB SSD and three traditional 1 TB HDDs to create a single disk group.

Allow enough capacity for the management and start point for 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.

Disk space must be scaled as necessary to accommodate full 100 workload VMs.

Because not all ROBO will be the same, 3TB of disk space provides a good starting point. Disk requirements will likely be higher depending on the workload disk size.

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

vSAN can be deployed as all-flash storage. 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 2. vSAN Mode Design Decision

Decision ID

Design Decision

Design Justification

Design Implication


Configure vSAN in hybrid mode.

Ensures a lower entry point for vSAN. Typically, virtual machines in ROBO do not require the performance of all-flash vSAN. Alternatively all-flash configuration could be used.

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