5.6.3.3 Flash-Based Devices

In vSAN 6.2 architecture flash-based devices can be used for both caching tier as well persistent capacity tier. In hybrid architectures, each vSphere host must have at least one flash-based caching—SAS, SATA, or PCI-e—to participate in the vSAN cluster. Flash-based devices provide both a write buffer and a read cache.

In a hybrid architecture, the larger the flash-based device capacity is per host, the larger the number of I/Os that can be cached and the greater the performance results that can be achieved. This scenario does not apply to the all-flash architecture.

The SSD used in a hybrid configuration is important because all reads and writes go to the SSD first. This is critical because it is what accounts for the speed that can be achieved with the vSAN solution.

In a hybrid system, the use of the SSD is split between a non-volatile write cache (30 percent) and a read buffer (70 percent). 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.

In all-flash architectures, each vSphere host must have at least one flash-based capacity—SAS, SATA, or PCI-e—marked as a capacity device and one for performance to participate in the vSAN cluster. The Virtual VSAN all-flash architecture is based on a two-tier model for performance and capacity. For an all-flash system, the cache is set to 100 percent writes because read performance is not a factor.

For endurance of the SSD used, standard industry write metrics are the primary measurements used to gauge the reliability of the drive. The measurement VMware uses has been updated to industry standard Terabytes Written (TBW) over the vendor’s warranty. Previously the specification used was Drive Writes per Day (DWPD).

VMware recommends selecting an endurance class based on the requirements of the environment. The different endurance classes are defined in the VMware Compatibility Guide for vSAN (https://www.vmware.com/resources/compatibility/search.php?deviceCategory=vsan) as shown in the following table.

Endurance Class	Terabytes Written in 5 Years (TBW)
Class A	>=365 TBW
Class B	>=1,825 TBW
Class C	>=3,650 TBW
Class D	>=7,300 TBW+

In addition to the generic classes, the following table lists the VMware recommendation for the endurance class based on the SSD tier as defined in the VMware Virtual SAN 6.0 Design and Sizing Guide (https://www.vmware.com/files/pdf/products/vsan/VSAN_Design_and_Sizing_Guide.pdf).

Endurance Class	SSD Tier	TB Writes Per Day	Terabyte Writes in 5 Years
Class A	All-Flash – Capacity	0.2	365
Class B	Hybrid - Caching	1	1,825
Class C	All-Flash – Caching (Medium Workload)	2	3,650
Class D	All-Flash – Caching (High Workload)	4	7,300

For optimal performance of vSAN, select a higher performance class of SSD. VMware defines classes of performance in the VMware Compatibility Guide for vSAN (https://www.vmware.com/resources/compatibility/search.php?deviceCategory=vsan) as shown in the following table.

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 – 100,000
Class F	100,000+

When designing a vSAN platform, it must be understood that a direct correlation exists between the SSD performance class and the level of vSAN performance. In general, the highest-performing hardware supports optimal performance of the solution. Cost, therefore, is the determining factor and might make a lower class of hardware more attractive, even though the performance or size might not be ideal.

The recommended strategy is to select an SSD size that is, at a minimum, 10 percent of the anticipated size of the consumed capacity storage, before the Number of Failures to Tolerate capability is considered. For instance, the SSD should be at least 100 GB if usage is estimated to be 1 TB of capacity storage, consumed in a 2-TB disk group.

Note In hybrid and all-flash architectures, flash-based caching devices do not contribute to the overall size of the distributed vSAN shared datastore. Because they are utilized for read and write caching, they count only toward the capacity of the vSAN caching tier or write buffer. In all-flash architecture, flash-based devices marked as capacity devices make up the size of the distributed vSAN datastore.