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Does your backup software do disk?

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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.

Where disk does help significantly is with recovering a large numbers of objects, such as a full file system restore, or in a disaster recovery (DR) scenario where data is likely to be stored on multiple tape volumes.

  • Improved reliability. One of the biggest complaints about tape is reliability. Tape isn't inherently unreliable, but because of differences in how it's managed, stored and handled, there can be wide variances and little predictability. This uncertainty is the real point of frustration. Disk, through RAID, can at least provide protection against media failures. Disk also benefits--from a reliability standpoint--from not being handled and transported.
  • Better manageability. This is the area where disk truly has the potential for the greatest impact. Many of the everyday problems of traditional backup are related to the idiosyncrasies involved in managing tape and tape devices. The failure of one tape device can have a cascading effect that impacts a number of backup jobs; managing and tracking tape cartridges is a process that invites all sorts of glitches. Disk, if integrated correctly, can smooth the process enormously, resulting in better reliability and faster backups and restores.

It's important to approach disk-based backup with a solid understanding of how disk impacts each of these areas in order to realize the potential benefits.

This was first published in September 2004

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