Design Considerations for Magnetic Disks in Virtual SAN

Plan the size and number of magnetic disks for capacity in hybrid configurations by following the requirements for storage space and performance.

SAS, NL-SAS, and SATA Magnetic Devices

Use SAS, NL-SAS, or SATA magnetic devices by following the requirements for performance, capacity, and cost of the Virtual SAN storage.

• Compatibility. The model of the magnetic disk must be certified and listed in the Virtual SAN section of the VMware Compatibility Guide.
• Performance. SAS and NL-SAS devices have faster performance than SATA disks.
• Capacity. The capacity of SAS, NL-SAS, and SATA magnetic disks for Virtual SAN is available in the Virtual SAN section of the VMware Compatibility Guide. Consider using a larger number of smaller devices instead of a smaller number of larger devices.
• Cost. SAS and NL-SAS devices are more expensive than SATA disks.

Using SATA disks instead of SAS and NL-SAS devices is justifiable in environments where capacity and reduced cost have higher priority than performance.

Magnetic Disks as Virtual SAN Capacity

Plan a magnetic disk configuration by following these guidelines:

• For better performance of Virtual SAN, use many magnetic disks that have smaller capacity.

  You must have enough magnetic disks that provide adequate aggregated performance for transferring data between cache and capacity. Using more small devices provides better performance than using fewer large devices. Using multiple magnetic disk spindles can speed up the destaging process.

  In environments that contain many virtual machines, the number of magnetic disks is also important for read operations when data is not available in the read cache and Virtual SAN reads it from the magnetic disk. In environments that contain a small number of virtual machines, the disk number impacts read operations if the Number of disk stripes per object in the active VM storage policy is greater than one.

• For balanced performance and predictable behavior, use the same type and model of magnetic disks in a Virtual SAN datastore.
• Dedicate a high enough number of magnetic disks to satisfy the value of the Primary level of failures to tolerate and the Number of disk stripes per object attributes in the defined storage policies. For information about the VM storage policies for Virtual SAN, see Using Virtual SAN Policies.