When deploying an S2D solution, a ratio can be configured between the number of cache devices and the number of capacity devices (1:2, 1:3, 1:4, 1:5, etc). This enables bonding a specific number of capacity devices to a cache device to ensure performance for the working set (=the data that is actively being used).
When designing an S2D cluster it must be ensured that the capacity of cache is at least 10% of raw data storage. This ensures that there is enough cache capacity to avoid read misses.
Because the cache data are replicated across nodes, even if a cache fails, the cache is not lost. Microsoft leverages RDMA to improve throughput between nodes. In case a cache device does fail, the related capacity devices are bound to another cache device in the same host. This is why Microsoft recommends at least two cache devices per node.