There is a direct correlation between the solid state disk (SSD) performance class and the level of vSAN performance. The highest-performing hardware results in the best performance of the solution. Cost is therefore the determining factor. A lower class of hardware that is more cost effective might be attractive even if the performance or size is not ideal.

For optimal vSAN performance, select class E or greater SSDs. For information on the different classes of SSD, see the VMware Compatibility Guide. 

SSD Performance Design Decision Background

Select a high class SSD for optimal vSAN performance. Before selecting a drive size, consider disk groups and sizing as well as expected future growth. VMware defines classes of performance in the VMware Compatibility Guide as follows.

Table 1. SSD Performance Classes

Performance Class

Writes Per Second

Class A

2,500 – 5,000

Class B

5,000 – 10,000

Class C

10,000 – 20,000

Class D

20,000 – 30,000

Class E

30,000 – 100,000

Class F

100,000 +

Select an SSD size that is, at a minimum, 10% of the anticipated size of the consumed HDD storage capacity, before failures to tolerate are considered. For example, select an SSD of at least 100 GB for 1 TB of HDD storage consumed in a 2 TB disk group.

Caching Algorithm

Both hybrid clusters and all-flash configurations adhere to the recommendation that 10% of consumed capacity for the flash cache layer. However, there are differences between the two configurations.

Hybrid vSAN

70% of the available cache is allocated for storing frequently read disk blocks, minimizing accesses to the slower magnetic disks. 30% of available cache is allocated to writes.

All-Flash vSAN

All-flash clusters have two types of flash: very fast and durable write cache, and cost-effective capacity flash. Here cache is 100% allocated for writes, as read performance from capacity flash is more than sufficient.

Use Class E SSDs or greater for the highest possible level of performance from the VMware vSAN volume.

Table 2. SSD Performance Class Selection

Design Quality

Option 1 Class E

Option 2 Class C

Comments

Availability

o

o

Neither design option impacts availability.

Manageability

o

o

Neither design option impacts manageability.

Performance

↑

↓

The higher the storage class that is used, the better the performance.

Recover-ability

o

o

Neither design option impacts recoverability.

Security

o

o

Neither design option impacts security.

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

Table 3. SSD Performance Class Design Decisions

Decision ID

Design Decision

Design Justification

Design Implication

CSDDC-PHY-STO-004

Use Class E SSDs (30,000-100,000 writes per second).

The storage I/O performance requirements for the management virtual machines dictate the need for at least Class E SSDs.

Class E SSDs might be more expensive than lower class drives.