The Primary level of failures to tolerate attribute from the VM storage policy does not impact the size of the cache. The flash caching device must provide at least 10 percent of the anticipated storage that virtual machines are expected to consume, not including replicas such as mirrors. This approach extends the life of capacity flash that might be less expensive and might have lower write endurance.ДЇor guidance on determining the cache ratio for all-flash clusters, refer to Designing vSAN Disk groups вЂ“ All Flash Cache Ratio Update. The write cache must be able to handle high write activities. In all-flash configurations, vSAN uses the cache layer for write caching only. However, when you configure multiple disk groups, the memory consumption of the hosts increases. Potentially improved performance if you deploy multiple disk groups that contain smaller flash caching devices.If a single caching device fails, fewer capacity devices are affected. The ratio between flash cache devices and disk groups is always 1:1.ДЄ configuration of multiple disk groups provides the following advantages: If you add more flash devices to increase the size of the cache, you must create more disk groups.Replacing a flash caching device is more complicated than replacing a capacity device because such an operation impacts the entire disk group.

Flash caching devices must have high write endurance.

Oversizing cache enables you to add more capacity to an existing disk group without the need to increase the size of the cache.

A higher cache-to-capacity ratio eases future capacity growth.

PCIe devices generally have higher cost than SSD devices.Дesign the configuration of flash cache for vSAN for write endurance, performance, and potential growth based on these considerations. For information about the write endurance class of SSD devices, see the vSAN section of the VMware Compatibility Guide. The write endurance of the SSD devices must meet the requirements for capacity or for cache in all-flash configurations, and for cache in hybrid configurations.ДЇor information about the write endurance requirements for all-flash and hybrid configurations, see the VMware vSAN Design and Sizing Guide. The maximum capacity that is available for PCIe devices is generally greater than the maximum capacity that is currently listed for SATA devices for vSAN in the VMware Compatibility Guide. PCIe devices generally have faster performance than SATA devices. The model of the SSD devices must be listed in the vSAN section of the VMware Compatibility Guide. Choosing Between PCIe or SSD Flash DevicesД¬hoose SSD flash devices according to the requirements for performance, capacity, write endurance, and cost of the vSAN storage. Plan the configuration of flash devices for vSAN cache and all-flash capacity to provide high performance and required storage space, and to accommodate future growth.

About cache device disk