The following shows the flow of allocating new pages and migrating them to the appropriate tier. The combination of determining the appropriate storage tier and migrating the pages to the appropriate tier is referred to as tier relocation.

Explanation of the relocation flow:

1. Allocate pages and map them to DP-VOLs

   Pages are allocated and mapped to DP-VOLs on an on-demand basis. Page allocation occurs when a write is performed to an area of any DP-VOL that does not already have a page mapped to that location. Normally, a free page is selected for allocation from an upper tier with a free page. If the capacity of the upper tier is insufficient for the allocation, the pages are allocated to the nearest lower tier. A DP-VOL set to a tier policy is assigned a new page that is based on the tier policy setting. The relative tier for new page allocations can be specified during operations to create and edit LDEVs. If the capacity of all the tiers is insufficient, an error message is sent to the host.

2. Gather I/O load information of each page

   Performance monitoring gathers monitoring information of each page in a pool to determine the physical I/O load per page in a pool. I/Os associated with page relocation, however, are not counted.

3. Create frequency distribution graph

   The frequency distribution graph, which shows the relationship between I/O counts (I/O load) and capacity (total number of pages), is created.

   You can use the View Tier Properties window to view this graph. The vertical scale of the graph indicates ranges of I/Os per hour and the horizontal scale indicates a capacity that received the I/O level. Note that the horizontal scale is accumulative.

   CautionWhen the number of I/Os is counted, the number of I/Os satisfied by cache hits are not counted. Therefore, the number of I/Os counted by Performance Monitoring is different from the number of I/Os from the host. The number of I/Os per hour is shown in the graph. If the monitoring time is less than an hour, the number of I/Os shown in the graph might be higher than the actual number of I/Os.

   Monitoring mode settings of Period or Continuous influences the values shown on the performance graph. Period mode will report the most recent completed monitor cycle I/O data on the performance graph. Continuous mode will report a weighted average of I/O data that uses recent monitor cycle data, along with historical data on the performance graph.

4. Determine the tier range values

   The page is allocated to the appropriate tier according to performance monitoring information. The tier is determined as follows.

   1. Determine the tier boundary

      The tier range value of a tier is calculated using the frequency distribution graph. This acts as a boundary value that separates tiers.

      The pages of higher I/O load are allocated to the upper tier in sequence. Tier range is defined as the lowest I/Os per hour (IOPH) value at which the total number of stored pages matches the capacity of the target tier (less some buffer percentage) or the IOPH value that will reach the maximum I/O load that the tier should process. The maximum I/O load that should be targeted to a tier is the limit performance value, and the rate of I/O to the limit performance value of a tier is called the performance utilization percent. A performance utilization of 100% indicates that the target I/O load to a tier is beyond the forecasted limit performance value.

      CautionThe limit performance value is proportional to the capacity of the pool volumes used in the tier. The total capacity of the parity group should be used for a pool to further improve the limit performance.
   2. Determine the tier delta values

      The tier range values are set as the lower limit boundary of each tier. The delta values are set above and below the tier boundaries (+10 to 20%) to prevent pages from being migrated unnecessarily. If all pages subject to tier relocation can be contained in the upper tier, both the tier range value (lower limit) and the delta value will be zero.

      

   3. Determine the target tier of a page for relocation.

      The IOPH recorded for the page is compared against the tier range value to determine the tier to which the page moves.

5. Migrate the pages

   The pages move to the appropriate tier. After migration, the page usage rates are averaged out in all tiers. I/Os which occur in the page migration are not monitored.