HDD is primarily meant as a high-capacity storage tier. To make optimal use of the flash layer in hybrid configurations, the Adaptive Optimization (AO) feature can be enabled. AO is a simple and smart sub-volume auto-tiering feature that transparantly moves more frequently accessed blocks to
the Tier-0 flash storage tier and keeps the infrequently accessed blocks on Tier-1 with lower performance and potentially lower-cost hard disk
drive (HDD) storage.
When using AO, new writes will always arrive at the fastest tier (Tier 0 = SSD). The algorithm logic ensures there is always a small amount of free space reserved on the fastest tier to accomodate new writes.