DS1621xs+ Maximum M.2 NVMe cache SSD?

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Very unlikely you would need more than 2TB of the cache. But I hope this helps:Max Read and Write cache -2x 1TB cache would require  416MB RAM   Read-only  832MB of RAMRead and Write cache -2x 2TB cache would require  832MB RAM   Read-only  1.7GB of RAMRead and Write cache -2x 4TB cache would require  1.7GB RAM   Read-only  3.3GB of RAMNone would need more then 4TB of cache, but you can have more if you add additional RAM stick,