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How the available Flash Pool cache capacity is calculated

Knowing the available cache capacity enables you to determine how many data SSDs you can add before reaching the limit, because Flash Pool cache capacity cannot exceed a platform-dependent limit for the system or HA configuration.

The current cache capacity is the sum of the "used size" capacity (as reported by the sysconfig -r command) of all of the data SSDs used in Flash Pool aggregates on the system. Parity SSDs are not included.

For systems in an HA configuration, the cache size limits apply to the HA configuration as a whole, and can be split arbitrarily between the two nodes, provided that the total limit for the HA configuration is not exceeded.

If Flash Cache modules are installed in a system, the available cache capacity for Flash Pool use is the Flash Pool cache capacity limit minus the sum of the Flash Cache module cache installed on the node. (In the unusual case where the size of the Flash Cache modules is not symmetrical between the two nodes in an HA configuration, the available Flash Pool cache capacity is decreased by the size of the larger Flash Cache module.)

For information about cache size limits, see the Hardware Universe.

Example calculation with Flash Cache modules

For an HA configuration composed of two storage controllers with a maximum cache capacity of 12 TB and 2 TB of Flash Cache installed on each node, the maximum Flash Pool aggregate cache capacity for the HA pair would be 12 TB minus 2 TB, or 10 TB.

Example calculation with asymmetrically sized Flash Cache modules

For an HA configuration composed of two storage controllers with a maximum cache capacity of 12 TB and 2 TB of Flash Cache installed on one node and 3 TB of Flash Cache installed on the other node, the maximum Flash Pool aggregate cache capacity for the HA pair would be 12 TB minus 3 TB, or 9 TB.