The HDDs in a vSAN environment have two different purposes, capacity and object stripe width.

Capacity

Magnetic disks, or HDDs, unlike caching-tier SSDs, make up the capacity of a vSAN datastore

Stripe Width

You can define stripe width at the virtual machine policy layer. vSAN might use additional stripes when making capacity and placement decisions outside a storage policy.

vSAN supports these disk types:

  • Serial Attached SCSI (SAS)

  • Near Line Serial Attached SCSI (NL-SCSI). NL-SAS can be thought of as enterprise SATA drives but with a SAS interface.

  • Serial Advanced Technology Attachment (SATA). Use SATA magnetic disks only in capacity-centric environments where performance is not prioritized.

SAS and NL-SAS get you the best results. This VMware Validated Design uses 10,000 RPM drives to achieve a balance between cost and availability. 

HDD Capacity, Cost, and Availability Background Considerations

You can achieve the best results with SAS and NL-SAS.

The VMware vSAN design must consider the number of magnetic disks required for the capacity layer, and how well the capacity layer will perform.

  • SATA disks typically provide more capacity per individual drive, and tend to be less expensive than SAS drives. However, the trade-off is performance. Due to lower rotational speeds (typically 7200RPM), SATA performance is not as good as that of SAS .

  • In environments where performance is critical, choose SAS magnetic disks instead of SATA magnetic disks.

Consider that failure of a larger capacity drive has operational impact on the availability and recovery of more components.

Rotational Speed (RPM) Background Considerations

HDDs tend to be more reliable, but that comes at a cost. SAS disks can be available up to 15,000 RPM speeds.

Table 1. vSAN HDD Environmental Characteristics

Characteristic

Revolutions per Minute (RPM)

Capacity

7,200

Performance

10,000

Additional Performance

15,000

Cache-friendly workloads are less sensitive to disk performance characteristics; however, workloads can change over time. HDDs with 10,000 RPM are the accepted norm when selecting a capacity tier.

For the software-defined storage module, VMware recommends that you use an HDD configuration that is suited to the characteristics of the environment.  If there are no specific requirements, selecting 10,000 RPM drives achieves a balance between cost and availability.

Table 2. HDD Selection Design Decisions

Decision ID

Design Decision

Design Justification

Design Implication

ROBO-PHY-STO-005

Use 10,000 RPM HDDs for the Consolidated cluster.

10,000 RPM HDDs achieve a balance between performance and availability for the VMware vSAN configuration.

The performance of 10,000 RPM HDDs avoids disk drain issues. In vSAN hybrid mode, the vSAN periodically flushes uncommitted writes to the capacity tier. 

Slower and potentially cheaper HDDs are not available.