In a VMware vSAN configuration, the SSDs are used for the vSAN caching layer for hybrid deployments and for the capacity layer for all flash.

  • 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. However, many more write operations are held by the caching tier, thus elongating or extending the life of the SSD capacity-tier.

SSD Endurance 

This VMware Validated Design uses class D endurance class 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, however, Drive Writes per Day (DWPD) or Petabytes Written (PBW) are the measurements normally used.

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. TBW can be used for VMware vSAN 5.5, VMware vSAN 6.0, and VMware vSAN 6.5 and is reflected in the VMware Compatibility Guide.

The reasoning behind using TBW is that VMware provides the flexibility to use larger capacity drives with lower DWPD specifications.

If an SSD vendor uses Drive Writes Per Day as a measurement, you can calculate endurance in Terabytes Written (TBW) with the following equation. 

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

For example, if a vendor specified DWPD = 10 for an 800 GB capacity SSD, you can compute TBW with the following equation. 

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

That means the SSD supports 7300 TB writes over 5 years (The higher the TBW number, the greater the endurance class.).

For SSDs that are designated for caching and all-flash capacity layers, the following table outlines which endurance class to use for hybrid and for all-flash VMware 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

Note:

This VMware Validated Design does not use All-Flash vSAN.

Table 1. SSD Endurance Class Design Decisions

Decision ID

Design Decision

Design Justification

Design Implication

SDDC-PHY-STO-003

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

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

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