Dynamic Tiering uses buffer percentages to reserve pages for new page assignments and 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. After 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 illustrates the workflow of a new page assignment.

For a pool comprised of pool volumes from parity groups with accelerated compression enabled, the capacity of the parity group equivalent to 20% of the FMC tier is used as the compression buffer area. When free space other than the FMC tier is not available, pages are assigned to this buffer area just before the capacity depletes.