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There is obviously a big difference in price and manageability between a configuration that needs 12 drives and one that needs 100 drives. As disks get more capacity and more data is addressed, this difference only becomes more profound. A system that scales speed based on disk drive count will waste enormous amounts of capacity. It would also create a need to manage many more drives than necessary. It will clearly cost more. In addition, if data is spread across drives, accesses would be scattered across all disks and not optimized for read throughput. The store would be fragmented, so the read performance required for recovery or streaming a copy to tape would be compromised.
Modern LTO-4 drives require speeds approaching 60 MB/s per stream or they will shoe-shine and slow down dramatically. At 1 MB/sec per disk, the system would need at least 60 disks (30 TB with 500 GB disks, or 45 TB with 750 GB disks) to perform well enough on reads, not including additional disks for spares. To succeed, a dedupe system would need to overcome the challenges of comparison across such a large logical space: