Hitachi Dynamic Tiering (HDT) optimizes disk usage in multi-tier Hitachi Dynamic Provisioning pools by moving the most frequently referenced pages to the fastest disks in the pool. To determine where each 42MB page should go, HDT tracks the amount of I/O received by each page in a configurable period of time called a monitoring cycle. By default, HDT weighs recent access patterns more heavily in its page placement decisions, but long-term patterns (over hundreds of monitoring cycles) are also factored in. HDT generally does a good job of keeping the hottest pages on the fastest drives, but until recently, it could not react immediately to sudden workload changes. For example, what happens when an application needs to read data that hasn’t been accessed in 3 months, or even a year? In an HDT pool with default settings, such data would probably need to be fetched from the lowest-performing tier. HDT would register the sharply increased activity on the suddenly-hot pages, but couldn’t promote them until the end of the current monitoring cycle at the earliest.
The new Active Flash feature of HDT addresses this problem by introducing a new real-time monitoring algorithm, as well as new thresholds for prompt promotion and high-priority demotion. With Active Flash enabled, HDT no longer has to wait until the end of a monitoring cycle (as long as 24 hours) to optimize page placement. The result should be a more nimble product that better balances between long-term and immediate performance requirements. We set out to test whether the new Active Flash feature achieves these design goals. The test results can be found in Active Flash Performance for Hitachi Virtual Storage Platform G1000 and Active Flash Performance for Hitachi VSP Gx00 Models.