In a vSAN configuration, you use Solid-State Disks (SSDs) for the vSAN caching layer in hybrid deployments and for the capacity layer in all-flash deployments. You consider the endurance parameters, such as DWPD and TBW, of a certain SSD class to select best SSD devices according to the requirements for your environment.

Consider the following endurance criteria according to the vSAN configuration:

  • For a hybrid deployment, the use of the SSD is split between a non-volatile write cache (approximately 30%) and a read buffer (approximately 70%). As a result, the endurance and the number of I/O operations per second that the SSD can sustain are important performance factors.

  • For an all-flash model, endurance and performance have the same criteria. The caching tier performs many more write operations, as result, elongating or extending the life of the SSD capacity tier.

SSD Endurance 

This design uses Class D SSDs for the caching tier.  

SDDC Endurance Design Decision Background

For endurance of the SSDs used for vSAN, standard industry write metrics are the primary measurements used to gauge the reliability of the drive. No standard metric exists across all vendors. Drive Writes per Day (DWPD) or Petabytes Written (PBW) are normally used as measurements.

For vSphere 5.5, the endurance class was based on Drive Writes Per Day (DWPD). For VMware vSAN 6.0 and later, the endurance class has been updated to use Terabytes Written (TBW), based on the vendor’s drive warranty. For more information about which vSAN versions that support TBW, see VMware Compatibility Guide.

Using TBW provides the option to use larger capacity drives with lower DWPD specifications.

If an SSD vendor uses DWPD as a measurement, calculate endurance in TBW with the following equation:

TBW (over 5 years) = Drive Size x DWPD x 365 x 5

For example, if a vendor specified a DWPD of 10 for an 800 GB capacity SSD, you can compute TBW by using the following equation:

TBW = 0.4 TB X 10 DWPD X 365 days X 5 yrs
TBW = 7300 TBW

As a result, the SSD supports 7300 TB writes over 5 years. The higher the TBW number, the higher the endurance class.

For SSDs that are designated for caching and all-flash capacity layers, consider the following endurance specifications for hybrid and for all-flash vSAN. 

Endurance Class

TBW

Hybrid Caching Tier

All-Flash Caching Tier

All-Flash Capacity Tier

Class A

>= 365

No

No

Yes

Class B

>= 1825

Yes

No

Yes

Class C

>= 3650

Yes

Yes

Yes

Class D

)=7300

Yes

Yes

Yes

Table 1. Design Decisions on SSD Endurance Class

Decision ID

Design Decision

Design Justification

Design Implication

ROBO-PHY-STO-003

Use Class D (>= 7300TBW) SSDs for the caching tier.

If an SSD in the caching tier fails due to wear-out, the entire vSAN disk group becomes unavailable. The result is potential data loss or operational impact.

SSDs with higher endurance could be more expensive than lower endurance classes.