Your organization runs a file share on a rack-mount server that contains eight 2 TB 10 000-RPM SAS drives in a single RAID 6 virtual disk. The array's usable capacity is 12 TB, of which 80 percent is already occupied. Monitoring shows the data set is growing at a steady 4 percent per month. You must ensure the share has enough space to last the next 18 months while keeping the same RAID level and without exceeding the server's 12-drive backplane. During the next maintenance window, what is the minimum number of additional 2 TB disks you must install?
Replace all existing drives with larger-capacity disks and rebuild an eight-drive RAID 6 array.
The array currently holds 9.6 TB of data (12 TB × 0.80). Over 18 months that amount will grow by (1.04)^18 ≈ 2.03, reaching roughly 19.5 TB. A RAID 6 array provides usable capacity equal to (N − 2) × 2 TB, so each extra 2 TB drive adds exactly 2 TB of usable space. To raise usable capacity from 12 TB to at least 19.5 TB requires ⌈(19.5 − 12) / 2⌉ = 4 additional drives, bringing the array to 12 disks and 20 TB of usable space. Adding two or three drives would leave less than 19.5 TB, and a wholesale replacement with larger disks costs more than the minimum solution.
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Why does a RAID 6 array require at least 'N-2' drives for usable capacity?
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How is the future storage requirement over 18 months calculated in this scenario?
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What does adding more drives to the array do in terms of RAID 6 capacity?