If you have 20 TB of data to backup, I would say most data deduplication appliances on the market today can handle that without too much of a problem. When you start scaling to 50 TB, 100 TB, 1 PB of backup data, the dynamics really change. You have backups going on. You have recoveries going on. You have data being moved off to tape. It just becomes a much more complex, dynamic environment.
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Some of the technologies I personally think are enterprise-ready are Dilligent Technologies and Sepaton. Both companies have had products on the market for some time now, and they are both having pretty good success in the market.
Check out the entire Data Deduplication FAQ.