This read-only integer reports the amount of memory, in megabytes, allocated for the Fibre Channel array accelerator's write cache, noting that because data is mirrored the actual usable cache is half this shown value. An administrator uses it to understand the real write-caching capacity available to this array when planning for write-heavy workloads. Seeing this value at 256 MB would mean an admin should plan around roughly 128 MB of genuinely usable mirrored write cache, not the full 256 MB shown.