The flash devices are used for Read and Write caching. When enabling Storage Spaces Direct (S2D), the SSDs are bound to the HDDs in round robin fashion. If an SSD fails, the HDDs are bound to another SSD within the same node. The same applies to the combinations NVMe/SSD, PMEM/SSD and PMEM/NVMe.
The cache information is replicated accross the node in cache storage devices. If a node fails and is up again, the cache information are recovered from others nodes in the cluster.
The cache information stored in a flash device is replicated accross the nodes within the storage cluster to be able to sustain the loss of a flash device or an entire node. If a failed node is up again, the cache information is automatically recovered from the others nodes in the cluster.
Microsoft, S2D [SDS]/2019, Datacenter
Persistent Write Buffer, Reads/Writes, Data Availability