Prompt Promotion

A primary goal of HDT and active flash is to have the most frequently access pages in Tier 1. As the workload varies in both the frequency of access and the type of access, reads or writes, the threshold for moving pages from one tier to another changes. HDT generates a dynamic tier range value that is used to determine which pages need to be in Tier 1 and which need to be in a lower tier.

The active flash feature uses the Prompt Promotion threshold to determine if a page qualifies to be promoted to Tier 1. The active flash feature compares the recent access frequency of each page to the Prompt Promotion threshold to determine whether a page should be promoted to Tier 1. The Prompt Promotion threshold is a dynamic threshold that adjusts based upon changes in workload to make most efficient use of the SSD drives. If the recent access frequency for a page meets or exceeds the Prompt Promotion threshold, the page is relocated to Tier 1 without waiting for the next HDT relocation cycle.

Certain type of I/O benefit more from being served by flash media then others. To achieve the best performance gains for certain I/O, active flash gives read I/O greater weight than write I/O when calculating the total access frequency for a page.