Q: Should I use Windows 2012 R2 Storage Space SSD tiering or write-back cache?
A:R2 Storage Spaces includes two key technologies: tiering by separating hard disk drive (HDD) and solid state disk (SSD) storage in a Storage Pool, and write-back cache.
Tiering allows the most-used blocks to be moved to the SSD tier providing the best performance or even allowing files to be explicitly pinned to the SSD tier. This provides the best overall performance improvement.
To use the SSD tiering, there must be at least as many SSD drives with available space in the Storage Pool to meet the Storage Space desired type. For example if a two-way mirror is created, there must be two SSDs with required space. If a three-way mirror is created, then there must be three SSDs with the required amount of space.
Write-back cache is a defined size that's stored on SSD storage available in the pool. The write-back cache is used when write spikes occur by writing the spike data to the write-back cache in the SSD tier, which avoids causing an overall drop in performance during those spikes.
Ideally, both technologies should be used rather than picking one. The use of SSDs in the tiering provides the best overall performance, while the write-back cache provides protection from performance impact during write spikes.
The same SSD drives in the Storage Pool can be used as part of the SSD tier and as part of the write-back cache, provided you don't manually set a drive as a journal drive (Usage-Journal, used only for write-back cache, which is generally not recommended; however, it does mean any space created will automatically get a 1GB write-back cache).