Backup is a trade-off between price and speed. Tape is relatively cheap, but tape restores are slow. Disk-based backups offer quick and easy restores, but it's still expensive to store more than a few terabytes of backup data (only a few days of data for some companies) on disk. A new technology from Data Domain called Capacity Optimized Storage (COS) reduces the cost of disk-based backups to less than $1 per gigabyte. COS compresses data to make the best use of disk space. The technology uses two levels of compression that let it store only unique blocks of compressed data. In contrast to the 2:1 compression ratio that most backup vendors advertise, Data Domain claims a 20:1 ratio.
I recently visited Data Domain to watch a demonstration of the DD460 backup device. The DD460 is a 3U (2.25") rack-mountable server that you access as Network Attached Storage (NAS). It makes 4TB of physical storage available, and the drive shows a total size of 4TB in Microsoft Windows Explorer. As you store data to the device, however, the used space will far exceed 4TB, and the free space will decrease by less than the amount of data you copy to a disk. At a 20:1 compression ratio, you can store 80TB of data on a disk. If your data is already compressed, encrypted, or naturally random, you might not get 20:1 compression ratios. However, if your data has a high level of redundancy, you could potentially see an even greater compression ratio.
Data Domain's COS technology makes disk-based backup a realistic solution. The DD440 includes built-in differential replication technology for off-site storage, and because it's accessible as a NAS device, it will work with most backup software. If you haven't considered disk-based backup because of the cost, take a look at Data Domain.
PROS: Disk-based backup at tape-based prices; minimal bandwidth used for replication