BTW Compellents architecture implementation was designed to ingest data to spinning disk as quickly as possible hence the top tier was always SAS Raid 10. However with the introduction of SSD, this had some unexpected consequences since all net new writes were forced to the top tier which was now low capacity SSD. Given the limited SSD write optimization and the 100% write workload, this top tier needs to be SLC, which in turn has a relatively high cost and low capacity. That being the case there's a very real potential incoming data could overrun the SSD tier and It's actually the reason they now propose a mix of SLC.and MLC. The relatively small SLC tier ingests data and demotes it quickly to the larger MLC tier to get around this architectural limitation.
Some things are not as cut and dried as they appear, Compellent's SLC/MCL write/read optimized tiering isn't so much a feature as a fix