You have four servers that run Windows Server.

When a Storage Spaces Direct pool contains three distinct drive tiers, in this case SSD, NVMe, and HDD as shown in the exhibit, Storage Spaces Direct automatically designates only the single fastest media type present, NVMe, as the cache devices for the pool, leaving both the SSD and HDD drives entirely to provide usable pool capacity rather than splitting cache duty across more than one tier. The default cache behavior that Storage Spaces Direct applies then differs depending on which capacity tier is being cached: cached HDD capacity drives receive full read-and-write caching, because HDDs benefit the most from having both random read and random write operations absorbed and accelerated by the much faster flash-based cache tier sitting in front of them. Cached SSD capacity drives, on the other hand, receive write-only caching, since SSD read latency is already low enough on its own that adding read caching in front of it would provide little additional performance benefit while still consuming cache capacity and adding unnecessary wear to the cache devices. Combining these two distinct behaviors together, the documented default outcome for this specific three-tier deployment is NVMe-only caching devices with a combined cache behavior of read/write for the HDD capacity tier and write-only for the SSD capacity tier.