Does your backup software do disk?
				Here's how three leading backup software applications deal with disk. IBM Tivoli Storage Manager (TSM) was designed to include disk as a key element, using a hierarchical model that maps directly to the "disk as cache" approach. The normal process in TSM is to back up to disk and then migrate to tape in an automated manner. Multiple read and write access is permitted to disk storage pools. High-water marks identify capacity boundaries that will force data migration to the next level in the storage hierarchy (usually tape). Data is migrated from disk to tape on a scheduled basis outside the normal backup window. Because TSM is optimized for a continuous incremental model, hybrid backup approaches aren't applicable. Other disk-based approaches may be used as well, but none have the same degree of built-in support. Legato NetWorker has supported disk-based backup since Version 5; its file device type supports writing backup savesets to disk in essentially the same format as it writes to tape. Version 7 significantly enhances disk support with its DiskBackup Option (DBO), adding an advanced file type that allows simultaneous access. So, for example, a backup operation and a restore operation can occur at the same time on the same device. NetWorker also fully supports staging to disk with automatic migration to tape. eritas NetBackup has supported disk as a backup device since version 3.1; Version 5 added a new device type known as a Disk Staging Storage Unit (DSSU) that enables staging data to disk and migrating to tape as an automated process. Version 5 also added the ability to create synthetic full backups--an important capability when disk is used to increase the number of incremental backups between fulls, but full backup tapes are still needed to send off site. NetBackup also allows simultaneous access to disk devices.

For those struggling with nightly backups, the arrival of low-cost disk promises to be the greatest breakthrough for improving the process since the introduction of centralized, networked backup in the early 1990s. Done properly, incorporating disk into a comprehensive data protection plan will improve the performance of backup and restore, reduce day-to-day management problems and provide an enhanced degree of data protection. To realize these benefits, it's necessary to answer these questions:

Where disk can help
In survey after survey, users cite the same reasons for adopting disk as a backup medium: faster nightly backups, shorter restore times, better reliability of backup and restore and better overall manageability. Here's how disk can help realize these efficiencies.

• Faster nightly backups. The impact of disk on individual backup jobs can vary dramatically. There are many potential causes for bottlenecks in the backup process, many of which are unrelated to whether disk is part of the picture. If these issues are causing your backup problems, just adding disk won't help. However, disk can help most with performance problems, such as queued jobs unable to start because of resource constraints and file servers that have many small files that can't keep a tape drive streaming.
• Shorter restore times. In many environments, the majority of restore requests are for individual files. Recovery performance for a small number of files is usually not a problem in a tape environment--assuming, of course, that the actual tape media is available. The time to load, seek and recover a file from a tape, while certainly greater than with disk, is often within acceptable limits.