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And don't forget those other backups, including the network data management protocol (NDMP) and specialized approaches for network-attached storage (NAS), block, file and e-mail-based approaches, hot database backups which require other data structures such as archive and redo logs and other replication approaches that may use iSCSI or even FTP for file transfers. Why is this important? Because with a disk-based backup environment, it's necessary to consider and integrate these approaches in the same way a tape-based backup environment does. The storage for a disk-based environment can be a storage area network (SAN)-based storage device or a LAN-based NAS device. The process of ensuring appropriate bandwidth to the device and allocating and managing disk storage for backup can significantly affect network planning.

DR. Operational (day-to-day) and DR (full-site failure) approaches are often linked, especially if tape is incorporated into the overall DR strategy. Disk-based backup approaches insert a unique kink into DR strategies, which may include off-site rotation of backup and/or special DR tapes.

Portability of tape media is probably the biggest asset of a tape-based approach when the ultimate DR site is "to be determined" at the time of the disaster. Conversely, assuming the disk-based backup array is not affected during a DR event, shipping a backup array (or disks within the array) is questionable at best. Real-time or near real-time replication of a disk-based

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backup array to another site is also an option, but that requires a dedicated DR site and WAN capacity, as well as the additional expense of redundant disk to protect the primary site's backup disk.

For most environments, tape will continue to play a major role in DR. Disk-based backup can be a key factor in improving the off-site tape production process. Many organizations today don't follow best practices with regard to daily "tape cloning" for off-site storage simply because they lack the time and the number of tape drives needed to meet the demand. Disk reduces the number of tape drives needed, and because multiplexing isn't necessary in this scenario, data can be recovered from off-site tapes more quickly and easily.

Putting it all together
The future of backup is with disk. Just as today the thought of doing non-centralized backups on individual servers is an anomaly in any IT shop of reasonable size, in a few years, backing up directly to tape will also become a rarity. Managing the transition is today's challenge. Storage managers need to consider what to change and how to do it, and make those changes in the least disruptive manner possible. Having a clear understanding of all the moving parts will help you smooth the transition and realize the true benefits of disk-based backup.

This was first published in September 2004

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