When considering the cluster and disk group design, you have to decide on the vSAN datastore size, number of ESXi hosts per cluster, number of disk groups per ESXi host, and the vSAN policy.

VMware vSAN Datastore Size

The size of the VMware vSAN datastore depends on the requirements for the datastore. Consider cost versus availability to provide the appropriate sizing.

Table 1. VMware vSAN Datastore Design Decision

Decision ID

Design Decision

Design Justification

Design Implication

CSDDC-VI-Storage-SDS-005

Provide the consolidated cluster with a minimum of 8 TB of raw capacity for vSAN.

Virtual machines in the consolidated cluster that use vSAN require at least 8 TB of raw storage.

None

CSDDC-VI-Storage-SDS-006

On all vSAN datastores, ensure that at least 30% of free space is always available.

When vSAN reaches 80% usage, a rebalance task is started which can be resource-intensive.

Increases the amount of available storage needed.

Number of ESXi Hosts Per Cluster

The number of ESXi hosts in the cluster depends on these factors:

  • Amount of available space on the vSAN datastore

  • Number of failures you can tolerate in the cluster

For example, if the vSAN cluster has only 3 ESXi hosts, only a single failure is supported. If a higher level of availability is required, additional hosts are required. 

Cluster Size Design Background

Table 2. Number of Hosts Per Cluster

Design Quality

3 ESXi Hosts

32 ESXi Hosts

64 ESXi Hosts

Comments

Availability

↑↑

The more ESXi hosts in the cluster, the more failures the cluster can tolerate.

Manageability

The more ESXi hosts in the cluster, the more virtual machines can run in the vSAN environment.

Performance

Having a larger cluster can impact performance if there is an imbalance of resources. Consider performance as you make your decision.

Recoverability

o

o

o

Neither design option impacts recoverability.

Security

o

o

o

Neither design option impacts security.

  Legend: ↑ = positive impact on quality; ↓ = negative impact on quality; o = no impact on quality.

Table 3. Cluster Size Design Decision

Decision ID

Design Decision

Design Justification

Design Implication

CSDDC-VI-Storage-SDS-007

Configure the consolidated cluster with a minimum of 4 ESXi hosts to support vSAN.

Having 4 ESXi hosts addresses the availability and sizing requirements, and allows you to take an ESXi host offline for maintenance or upgrades without impacting the overall vSAN cluster health.

The availability requirements for the consolidated cluster might cause underutilization of the cluster ESXi hosts.

Number of Disk Groups Per ESXi Host

Disk group sizing is an important factor during volume design.

  • If more ESXi hosts are available in the cluster, more failures are tolerated in the cluster. This capability adds cost because additional hardware for the disk groups is required.

  • More available disk groups can increase the recoverability of vSAN during a failure.

Consider these data points when deciding on the number of disk groups per ESXi host:

  • Amount of available space on the vSAN datastore

  • Number of failures you can tolerate in the cluster

The optimal number of disk groups is a balance between hardware and space requirements for the vSAN datastore. More disk groups increase space and provide higher availability. However, adding disk groups can be cost-prohibitive.

Disk Groups Design Background

The number of disk groups can affect availability and performance. 

Table 4.  Number of Disk Groups Per ESXi Host

Design Quality

1 Disk Group

3 Disk Groups

5 Disk Groups

Comments

Availability

↑↑

The more ESXi hosts in the cluster, the more failures the cluster can tolerate.

This capability adds cost because additional hardware for the disk groups is required.

Manageability

o

o

o

The more ESXi hosts in the cluster, more virtual machines can be managed in the vSAN environment.

Performance

o

↑↑

If the flash percentage ratio to storage capacity is large, vSAN can deliver increased performance and speed.

Recoverability

o

↑↑

More available disk groups can increase the recoverability of vSAN during a failure.

Rebuilds complete faster because there are more places to place data and to copy data from.

Security

o

o

o

Neither design option impacts security.

Legend: ↑ = positive impact on quality; ↓ = negative impact on quality; o = no impact on quality.

Table 5. Disk Groups Per ESXi Host Design Decision

Decision ID

Design Decision

Design Justification

Design Implication

CSDDC-VI-Storage-SDS-008

Configure vSAN with a minimum of one disk group per ESXi host.

Single disk group provides the required performance and usable space for the datastore.

Losing an SSD in an ESXi host takes the disk group offline.

Using two or more disk groups can increase availability and performance.