About active flash
Active flash
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.
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.
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.
The following diagram shows the differences in the way pools are managed between
Dynamic Provisioning,
Dynamic Tiering, and
active flash
Page relocation by active flash
The active flash feature identifies the frequently accessed pages by counting the number of I/Os to specific pages. As shown in the following figure, pages that are accessed frequently are promoted to tier 1, and pages whose latest access frequency is low are allocated to lower tiers.
Functions overview for active flash and Dynamic Tiering
Tier management is performed by both active flash and Dynamic Tiering. The differences in supported functionality are included in the table below.
|
Category |
Functions |
active flash |
Dynamic Tiering |
|
Initial page allocation |
Assigning new pages to the write data of the host |
Supported |
Supported |
|
Monitoring of performance |
Monitoring tiers based on the specified cycle time |
Supported |
N/A |
|
Tier relocation |
Promoting pages to the tier which is determined by the scheduled performance monitoring |
Supported |
Supported |
|
Promoting pages from the tier 2 or 3 to tier 1, the pages where the latest access frequency is suddenly high |
Supported |
N/A | |
|
To maintain capacity in the tier 1, demoting pages from the tier 1 to tier 2 or 3, the pages where the latest access frequency is low |
Supported |
N/A |
Active flash workflow
The active flash feature of Dynamic Tiering can be set up using either Hitachi Device Manager - Storage Navigator or Command Control Interface.
The following illustration shows the workflow for a Storage Administrator to set up active flash on the storage system. As shown in the illustration, Hitachi Device Manager - Storage Navigator and Command Control Interface have different workflows. The details about how to set up active flash using Hitachi Device Manager - Storage Navigator are covered in subsequent topics. For details about how to set up active flash using Command Control Interface, see the Command Control Interface Command Reference and Command Control Interface User and Reference Guide. Use Hitachi Device Manager - Storage Navigator to create pools and DP-VOLs. 
- In Command Control Interface, when creating a pool, you cannot enable Multi-Tier Pool and cannot register multiple media as pool-VOLs. Before making tiers, enable Multi-Tier Pool.
- Enabling Multi-Tier Pool from Command Control Interface automatically sets Tier Management to Manual. To change Tier Management to Auto, you must do this in Hitachi Device Manager - Storage Navigator.