As mentioned at the start of the chapter, the tiering functionality should be thought of as tiering and not a cache. The reason behind this statement is that the act of promotions has a detrimental effect to cluster performance when compared with most caching solutions, which do not normally degrade performance if enabled on non-cacheable workloads. The performance impact of promotions are caused for two main reasons. First, the promotion happens in the I/O path; the entire object to be promoted needs to be read from the base tier and then written into the top tier before the I/O is returned to the client.
Second, this promotion action will likely also cause a flush and an eviction, which causes even more reads and writes to both tiers. If both tiers are using 3x replication, this starts to cause a large amount of write amplification for even just a single promotion...