Buffer area of a tier
Dynamic Tiering uses buffer percentages to reserve pages for new page assignments and to allow the tier relocation process. Areas necessary for processing these operations are distributed corresponding to settings used by Dynamic Tiering . The following describes how processing takes place to handle the buffer percentages.
Buffer space: The following table shows the default rates (rate to capacity of a tier) of buffer space used for tier relocation and new page assignments, listed by drive type.
|
Drive type |
Buffer area for tier relocation |
Buffer area for new page assignment |
Total |
|---|---|---|---|
|
SSD |
2% |
0% |
2% |
|
Non-SSD |
2% |
8% |
10% |
New page assignment: New pages are assigned based on a number of optional settings. Pages are then assigned to the next lower tier, leaving a buffer area (2% per tier by default) for tier relocation. When 98% of capacity of all tiers is assigned, the remaining 2% of the buffer space is assigned from the upper tier. The buffer space for tier relocation is 2% in all tiers. The following shows the workflow of new page assignment.
Workflow of a new page assignment
For a pool using pool volumes assigned from accelerated compression-enabled parity groups, the capacity of parity group equivalent to 20% of the FMC tier is used as the compression buffer area. When there is no free space aside from the FMC tier, pages are assigned to this buffer area just before the capacity is deleted.
