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How the available Flash Pool cache capacity is calculated

Flash Pool cache capacity cannot exceed a platform-dependent limit for the node or HA configuration. Knowing the available cache capacity enables you to determine how much cache capacity you can add to your Flash Pool aggregates before reaching the limit.

The current Flash Pool cache capacity is the sum of all of the Flash Pool caches (as reported by the storage aggregate show command). For nodes using SyncMirror, including MetroCluster configurations, only the Flash Pool cache of one plex counts toward the cache limit.

If Flash Cache modules are installed in a node, 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 nodes in an HA configuration, the cache size limit applies 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.

For nodes in a MetroCluster configuration, the cache size limit applies to the configuration as a whole, but it cannot be split arbitrarily between the nodes as it can for an HA configuration. This means that the limit for each node in the MetroCluster configuration is one quarter of the total cache size limit. If Flash Cache is present on any of the nodes, then the size of the largest Flash Cache module must be subtracted from the overall limit before the per-node limit is calculated.

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.

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 cache capacity for the HA pair would be 12 TB minus 3 TB, or 9 TB.

Calculation for a MetroCluster configuration

For a MetroCluster configuration composed of four storage controllers with a maximum cache capacity of 12 TB, the maximum Flash Pool cache capacity for each node would be 12 TB divided by four, or 3 TB.

Calculation for a MetroCluster configuration with Flash Cache modules

For a MetroCluster configuration composed of four storage contollers with a maximum cache capacity of 12 TB and 2 TB of Flash Cache installed on one node, the maximum Flash Pool cache capacity for each node would be 12 TB minus 2 TB divided by four, or 2.5 TB.