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

vSAN Datastore Size

The size of the VMware vSAN datastore depends on the requirements for the datastore. To determine the appropriate size consider cost versus availability.

Table 1. VMware vSAN Datastore Design Decision

Decision ID

Design Decision

Design Justification

Design Implication

ROBO-VI-STORAGE-SDS-005

Provide the consolidated cluster with enough vSAN space to run management and workloads virtual machines with FTT=1.

Management virtual machines require a maximum of 6 TB of storage when using FTT=1.

You must provide enough storage for all virtual machines.

Monitoring and capacity management are critical and must be performed proactively.

ROBO-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 re-balance task is started which can be resource intensive.

Increases the amount of available storage needed.

Monitoring and capacity management are critical and must be performed proactively.

Number of Hosts Per Cluster

The number of 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 you require a higher level of availability, you must use additional hosts. 

Cluster Size Design Background

Table 2. Number of Hosts Per Cluster

Design Quality

4 Hosts

32 Hosts

64 Hosts

Comments

Availability

↑↑

The more hosts that are available in the cluster, the more failures the cluster can tolerate.

Manageability

The more hosts in the cluster, the more virtual machines can be in the VMware 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

ROBO-VI-STORAGE-SDS-007

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

Having 4 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.

Depending on the workloads deployed more hosts may need to be added for both compute and storage resources.

Monitoring and capacity management are critical and must be performed proactively.

Number of Disk Groups Per Host

Disk group sizing is an important factor during volume design.

  • If more 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 VMware vSAN during a failure.

Consider these data points when deciding on the number of disk groups per 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 Host

Design Quality

1 Disk Group

3 Disk Groups

5 Disk Groups

Comments

Availability

↑↑

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

Manageability

o

o

o

If more hosts are 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, the 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 Host Design Decision

Decision ID

Design Decision

Design Justification

Design Implication

ROBO-VI-STORAGE-SDS-008

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

Single disk group provides the required performance and usable space.

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

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

Depending on the workloads deployed creating a second disk group may be required.