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Looking at the numbers
It's generally accepted that disk storage has a lower TCO vs. tape-based storage due to its ease of use and lower management overhead. Disk is also multifunctional compared to single-use tape. However, the inconvenience of having to manage and move tapes is greatly offset by the lower cost of the media. In our example (50GB daily for incrementals x four incrementals/week x four week retention + 250GB weekly for cumulative incremental x three weeks/month x three month retention=3TB) disk storage requirements will increase by 3TB. Consequently, there's a balance between what data is backed up to disk and what data is sent to tape.
The cost of using both disk and tape for backups is not only cost-effective, but provides a higher level of availability and business continuance (see "Using disk and tape for incremental backups," this page). Specific to database environments, another trade-off is the management of database transaction logs. Normally, full database backups truncate log files after they're copied to tape. Any logs that have already committed their transactions to the database are either archived or deleted. As indicated in Rotation schedule with incremental backups", full backups are performed at the end of every month, allowing the database log files to remain on disk. As a result, database log files will be monitored and managed.
Today, most organizations perform multiple backups during the day. Generally, this is accomplished through snapshot copies generated from the file system (Network Appliance's WAFL, Veritas VxFS), volume manager and utilities (Veritas VxVM, Sun StorEdge Instant Image) or through storage arrays (Sun/HDS ShadowImage, HP Business Copy VA). Either way, the data is readily available and safely stored on disk. Most organizations will then make a tape copy from the end-of-day snapshot--or better yet, replicate the snapshot to an off-site facility where tape copies are created.
Generating incremental snapshots during the day provides an extra level of data availability and is highly recommended. These point-in-time copies, however, don't replace the need of using third-party backup software. Commercial backup software provides proper auditing, management and configuration controls as well as extensive recoverability options for data protection requirements.
Mixing disk and tape for backups provides better flexibility, improved performance and enhanced business continuance. Disk backups are based on open standards and seamlessly integrate into an existing storage network infrastructure. The costs associated with using disk for backup purposes needs to be balanced against service level agreements and budgetary constraints. It's best for organizations to invest in extra disk capacity when budgeting, designing or building a SAN. Investing in additional capacity during a SAN deployment offers a lower cost of entry for the opportunity of incorporating disk storage into backup processes. In the long run, both disk and tape offer a cost-effective, highly available data protection solution.
This was first published in March 2003