What's better for backup: tape or disk?


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Recently, an increasing number of clients have been asking me about replacing tape with disk in some--or all--of their backup infrastructure. Until recently, the cost of tape was far less per unit of storage than any other media. It is easily transportable, making off-site storage possible. In addition, newer tape technologies have increased both speed and capacity dramatically. So why do people want to replace it?

Typical complaints about tape focus on four areas:

  • Performance. Because it's serial, tape is perceived--rightly or wrongly--to be slower than disk.
  • Reliability. Everyone who has dealt with backups has experienced tape failure: they wear out, get mishandled or just go bad.
  • Handling. Removing cloned tapes from a library, adding scratch tapes and cataloging bar codes--tape handling can be a nightmare, especially given the ever-increasing number of tapes.
  • Cost. Tape media costs and tape off-site storage costs are budget line items that continue to soar.
However, upon closer analysis, one finds that these really aren't tape problems per se. They are backup management problems: people/process/policy problems that are the result of inadequate backup architectures, lax operational procedures and poor data policy management. While some characteristics of tape can exacerbate these problems, tape itself is not the root cause.

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So, if you're experiencing one or more of these problems regularly in your environment, don't bet on disk as the panacea for backup.

Disk-based backup options
Low-cost disk subsystems Traditional SAN or NAS-based storage arrays Network Appliance NearStore, StorageTek BladeStore, Nexsan InfiniSAN D2D
Virtual tape devices RAID disk arrays that emulate a tape library Quantum DX30, Neartek VSE2
Content-addressable storage systems Highly scalable, multiple low-profile units in a Redundant Array of Independent Nodes (RAIN) architecture EMC Centera, Avamar Axion

Disk choices
Using disk as a backup device isn't really a new concept. In the mainframe world, virtual tape systems utilizing disk drives as a cache and staging area for tape devices have been in widespread use for a number of years. In open systems, disk has been involved with backup in at least three ways. First, business continuance mirroring products such as EMC's TimeFinder have played an increasingly important role in improving backup performance and offloading production servers. Also, some backup products, such as IBM Tivoli Storage Manager, have utilized the concept of intermediate disk storage pools on backup servers as the target repository for nightly backups, thereby reducing contention for tape drives. Finally, virtually all backup products support backup to a disk device instead of--or in addition to--tape. In all of these cases, however, disk has been used as a temporary repository--the backup data ultimately ends up on tape.

Recently, a number of new products have emerged based on low-cost storage devices, most notably ATA disks. These and other devices are presenting new options that provide some interesting options for designing a backup architecture. These new devices tend to fall into one of three categories: low-cost disk subsystems, virtual tape devices and content-addressable storage systems (see "Disk-based backup options," this page).

Low-cost disk subsystems are probably the most recognizable type of device. These are typically ATA-based systems that share many characteristics with traditional SCSI and Fibre Channel (FC) disk systems. The major differentiating factors between these and their higher-priced siblings are performance and cost. They are positioned in the market as near-online storage devices. Because their cost per megabyte is approaching tape, and they provide some level of RAID protection, it has become feasible to consider their use in backup environments. Additionally, some vendors offer features such as replication and/or snapshot capabilities to further enhance their capabilities.

This was first published in March 2003

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