Hardware Considerations
The choice of physical hardware to be used in a Virtual SAN environment is of upmost importance to the configuration. Although this solution is agnostic to the hardware being used, hardware from the VMware Compatibility Guide for Virtual SAN (https://www.vmware.com/resources/compatibility/search.php?deviceCategory=vsan) must be used for the following components:
Solid state disks (SSDs)
Magnetic hard drives (HDDs)
I/O controllers
A number of vendors produce a Virtual SAN Ready Node as listed in the VMware Virtual SAN Compatibility Guide. In many cases, this is a more appealing solution than individual verification of the components listed above.
| VMware Engineering and VMware Technical Support will not provide any support (including best-effort support) for environments that do not have hardware listed in the VMware Compatibility Guide for Virtual SAN (https://www.vmware.com/resources/compatibility/search.php?deviceCategory=vsan). | |
|---|---|
Solid State Disks
The SSDs used in a Virtual SAN configuration are of vital importance to the configuration because all reads and writes go to the SSD first. The SSDs are used for the caching layer of Virtual SAN, not for storage. The use of the SSD is split between a non-volatile write cache (approximately 30 percent) and a read buffer (approximately 70 percent). As a result, the endurance and the number of I/O operations per second that the SSD is capable of sustaining are important factors in the performance of the solution.
For endurance of the SSD used for Virtual SAN, standard industry write metrics are the primary measurements used to gauge the reliability of the drive. There is no standard metric across all vendors, however. Drive Writes per Day (DWPD) or Petabytes Written (PBW) are the measurements normally used.
VMware endurance requirements for SAS and SATA SSDs in a five-year lifespan are as follows:
The drive must support at least 10 full DWPD.
The drive must support random write endurance up to 3.5 PB on 8 KB transfer size per NAND module, or up to 2.5 PB on 4 KB transfer size per NAND module.
VMware endurance requirements for PCIe SSDs are as follows:
The drive must support at least 10 full DWPD.
The drive must support random write endurance up to 3.5 PB on 8 KB transfer size per NAND module, or up to 2.5 PB on 4 KB transfer size per NAND module.
| All drives listed in the VMware Compatibility Guide for Virtual SAN (https://www.vmware.com/resources/compatibility/search.php?deviceCategory=vsan) meet this requirement. | |
|---|---|
In addition, for optimal performance of Virtual SAN, select a higher class of SSD. VMware also defines classes of performance in the VMware Compatibility Guide for Virtual SAN (https://www.vmware.com/resources/compatibility/search.php?deviceCategory=vsan) as follows:
Table 48. SSD Performance Classes
| 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+ |
A direct correlation exists between the SSD performance class and the level of Virtual SAN performance. In general, as with most hardware, the highest-performing hardware supports optimal performance of the solution. Cost, therefore, is the determining factor, and thus, might make a lower class of hardware more attractive, even though the performance or size might not be ideal.
| A good criterion is to select an SSD size that is, at a minimum, 10 percent of the anticipated size of the consumed HDD storage capacity (before failures to tolerate are considered). | |
|---|---|
The recommended configuration when using the Software-Defined Storage module is to use Class E SSDs in the configuration to gain the highest level of performance possible from the Virtual SAN volume.
Table 49. SSD Performance Class Selection
| 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. |
| Recoverability | 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.
Table 50. SSD Performance Class Selection – Design Decisions
| Decision ID | Design Decision | Design Justification | Design Implication |
|---|---|---|---|
| For this design, <Customer> has made the following decisions listed in this table. | |||
Magnetic Hard Disk Drives
The HDDs in a Virtual SAN environment are used primarily to provide data storage capacity. They also are a determining factor in the available stripe width for VM storage policies. As a result, if a specific stripe width is required, you must make sure that a particular stripe width is available across all hosts in the cluster to meet the requirements. In addition, if the VM has a high failure to tolerate setting, additional HDDs are necessary because each component must be replicated to meet this requirement. This topic is discussed further in Section 8.3, Virtual SAN Datastore Characteristics and Section 5.3.3, Datastore Sizing. As a result, the speed of the HDDs should be chosen to meet the environmental characteristic for which the cluster is designed. VMware defines HDD characteristics and speed in the following table.
Table 51. Virtual SAN HDD Environmental Characteristics
| Characteristic | Revolutions per Minute |
|---|---|
| Capacity | 7200 |
| Performance | 10,000 |
| Additional Performance | 15,000 |
For the Software-Defined Storage module,VMware recommends that you use an HDD configuration suited to the characteristics of the environment being designed. If high performance is not required, a lower cost disk will allow a higher number of failures to be tolerated. If there are no specific requirements, selecting 10,000 RPM drives achieves a balance between cost and availability.
| VMware recommends that you avoid mixing and matching HDD speeds to achieve a blend of different characteristics in the environment, because there is only a single LUN per array. | |
|---|---|
Table 52. HDD Characteristic Selection
| Design Quality | Option 1 7200 RPM | Option 2 10,000 RPM | Option 3 15,000 RPM | Comments |
|---|---|---|---|---|
| Availability | ↑ | ↓ | ↓ | Less expensive disks make it easier to achieve a larger number of failures to tolerate without a large cost being incurred. Therefore, slower disks are an appealing option for an environment in which availability is important. |
| Manageability | o | o | o | No design option impacts manageability. |
| Performance | ↓ | ↑ | ↑↑ | In a Virtual SAN environment, performance is best when using high-RPM HDDs. However, a high-performing SDD will impact performance more than a high-RPM HDD. |
| Recoverability | o | o | o | No design option impacts recoverability. |
| Security | o | o | o | No design option impacts security. |
Legend: ↑ = positive impact on quality; ↓ = negative impact on quality; o = no impact on quality.
Table 53. HDD Selection – Design Decisions
| Decision ID | Design Decision | Design Justification | Design Implication |
|---|---|---|---|
| For this design, Customer> has made the following decisions listed in this table. | |||
| DD0069 |
I/O Controllers
The I/O controllers are as important to a Virtual SAN configuration as the selection of disk drives is. Virtual SAN supports SAS, SATA, and SCSI adapters in either pass-through or RAID-0 modes. These are the only modes that are supported. Many storage adapters support both modes. However, performance is highly dependent on the I/O controller.
Consider the following when selecting a storage adapter:
What modes does it support? (RAID-0, pass-through, or both)
In RAID-0 mode SSD performance should be a factor reviewed
RAID-0 mode also comes with operational overhead that can impact performance
Storage controller interface speed
Number of devices supported for the controller
Controller queue depth is important for performance. Ideally, you should select a queue depth of 256 or higher. This is a large determining factor for the end performance of virtual SAN.
| Regardless of the choice made, Virtual SAN requires complete control of the drives. Performance between pass-through and RAID-0 modes is generally very similar for most interfaces. | |
|---|---|
The recommended configuration for the Software-Defined Storage moduleis to use I/O controllers suited for the design characteristics of the environment including:
Model of the SSDs
Model of the HDDs
Queue depth of the controller
Number of disks (and corresponding disk groups) being configured
Table 54. HDD Characteristic Selection
| Design Quality | Option 1 Higher Queue Depth | Option 2 Larger Number of Drives | Comments |
|---|---|---|---|
| Availability | o | ↑ | A large number of drives increases availability by providing a greater failure to tolerate threshold. |
| Manageability | o | o | Neither design option impacts manageability. |
| Performance | ↑ | o | The higher the queue depth on the controller, the higher the performance of the Virtual SAN volume. |
| Recoverability | 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.
Table 55. I/O Controller Selection – Design Decisions
| Decision ID | Design Decision | Design Justification | Design Implication |
|---|---|---|---|
| For this design, <Customer> has made the following decisions listed in this table. | |||
Host Memory Requirements
The Virtual SAN memory requirements are defined based on the number of disk groups and disks that the hypervisor manages. To support the maximum number of disk groups, 32 GB of RAM is required.
| See Section 8.3, Virtual SAN Datastore Characteristics for more information about disk groups, including design and sizing guidance. | |
|---|---|
The recommended configuration for the Software-Defined Storage module is to use 256 GB of RAM for the ESXi hosts. This is enough memory to support the largest disk group configuration allowed.
| A best practice is to assign a consistent amount of RAM to each host in the cluster. | |
|---|---|
Table 56. Host Memory Selection
| Design Quality | Option 1 6 GB RAM | Option 2 16 GB RAM | Option 3 32+ GB of RAM | Comments |
|---|---|---|---|---|
| Availability | ↓ | ↑ | ↑↑ | More RAM allows for more disk groups and disks, providing a greater-failures-to-tolerate threshold. |
| Manageability | o | ↑ | ↑↑ | With more RAM in the system, more VMs can be managed, enabling greater flexibility and consolidation ratios. |
| Performance | o | o | o | The amount of RAM does not significantly impact performance of the Virtual SAN environment. Most configurations will have more than 32 GB of RAM per host. |
| Recoverability | o | o | o | Neither design option impacts recoverability. |
| Security | o | o | o | Neither design option impacts security. |
Legend: ↑ = positive impact on quality; ↓ = negative impact on quality; o = no impact on quality.
Table 57. Host Memory Selection – Design Decisions
| Decision ID | Design Decision | Design Justification | Design Implication |
|---|---|---|---|
| For this design, <Customer> has made the following decisions listed in this table. | |||
Host CPU Overhead
Virtual SAN does not have specific CPU requirements because it introduces less than 10 percent CPU overhead. This impacts available resources in high consolidation ratio workloads and CPU-intensive applications.
When designing and sizing the environment, take into account the projected resource requirements for the environment, so that performance will not suffer due to a lack of resources or over-commitment of resources.