Active flashActive flash for mainframe

The active flash feature of Dynamic Tiering monitors page accesses over a set time frame and attempts to keep the most frequently accessed pages in Tier 1.

The active flash feature monitors a page's access frequency level real time and promotes pages that suddenly became busy from a slower media to high-performance flash media, in real-time.

The active flash feature can be enabled on any Dynamic Tiering pool as long as you have SSD drives in Tier 1 of the pool. No special configuration beyond what is needed for active flash is required.

Prompt Promotion

A primary goal of Dynamic Tiering 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. Dynamic Tiering 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 compares the recent the 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 Dynamic Tiering 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.

High Prioritized Demotion

In order to be certain that there is always some room for active flash to do Prompt Promotion of pages to Tier 1, High Prioritized Demotion is used to demote pages out of Tier 1. Pages that have the lowest IOPH are candidates for High Prioritized Demotion. Similar to Prompt Promotion, High Prioritized Demotion does not wait for the current Dynamic Tiering cycle to end to make relocation decisions.

Page demotion is only triggered when:

  • Tier 1 free capacity is depleted
  • performance utilization reaches 80%

Peak performance utilization is predefined for a particular media.

Performance utilization of a tier is the maximum amount of I/O it can receive. The maximum I/O load that should be targeted to a tier depends upon the media type used to make the tier. A performance utilization of 100% means that the tier is receiving the maximum amount of I/O it can sustain. When performance utilization reaches about the 60% level, response time to the particular media becomes noticeably slower. For more information about performance utilization, see Tier relocation workflow.

The following diagram shows the differences in the way pools are managed between Dynamic Provisioning, Dynamic Tiering, and active flash

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  • Dynamic Tiering

    Hitachi Dynamic Tiering (HDT) simplifies storage administration by automatically optimizing data placement in 1, 2 or 3 tiers of storage that can be defined and used within a single virtual volume. Tiers of storage can be made up of internal or external (virtualized) storage, and use of HDT can lower capital costs. Simplified and unified management of HDT allows for lower operational costs and reduces the challenges of ensuring applications are placed on the appropriate classes of storage.