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.
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.
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.
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. |