Software-defined storage is a key technology in the SDDC. This design uses VMware Virtual SAN (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 disk basis through the use of storage policies.

Requirements and Dependencies

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

  • Minimum of 3 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 disks 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 HHDs, for hybrid, or two additional flash devices for an all-flash configuration.

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

Table 1. vSAN Physical Storage Design Decision 

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.

Allows 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 workload VMs.

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. If required an all-flash configuration can be used.

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