The trend towards disk backup would seem to herald the death of tape. But the old technology has proved remarkably resilient. Timing is everything, and deciding when to make the switch is not as simple as it would at first seem.

The shifting emphasis towards disk backup is a relatively recent phenomenon, but growth has been driven by long-standing issues for systems administrators. In fact, it's the natural convergence of three of their most basic concerns:

Technology: Thanks to the advent of Serial ATA, the cost per gigabyte of the hard disk has fallen dramatically. SCSI and Fibre Channel disks now offer consistently excellent, if relatively expensive, performance.

Operational problems: The time available to save data is becoming increasingly tight. But the quantity of business data that needs saving is growing inexorably. Unfortunately, tape manufacturers haven't succeeded in increasing performance to keep up with this growth.

User unhappiness: Users frequently complain about the fact that tape devices attached to the infrastructure are neither sufficiently fast nor reliable. But these concerns are nt always enough to sway a business decision.

Disk backup
In the traditional backup environment, data flows from the production server to the space dedicated to disk backup. Once there, it is archived, modified and cleaned up - depending on the policy of the systems administrator - for the purpose for which it is intended.

The data is then sent out to tape libraries to be permanently archived on an offline, or even offsite, support system.

This strategy of high-speed backup and recovery is called disk-to-disk-to-tape (DDT). The approach has its advantages: it reduces time generally, but specifically it significantly cuts the time spent cleaning up files.

Hard disks have a regulating role, managing quick access to information. But you cannot say the same for tape units.

As far as security is concerned, backup disk solutions now offer Raid functionality which provides added protection to tapes, ensuring the integrity of data even where one or more disk drives are lost. And because the disks are replaceable, there is no chance of operational meltdown.

Not surprisingly, major tape library vendors such as StorageTek, HP, Quantum and Overland have increased their portfolio of products to include disk-based backup products.

EMC has also recently released an analogue offering based on the Clariion platform, called Clariion DL (disk library). It offers a backup-to-disk tool that doesn't require changes to existing applications.

IBM, with Data Retention 450, is also pushing to provide an integrated, all-encompassing product for business data retention.

But how do you manage these new backup-to-disk tools? Obviously, the various software producers (Veritas, CA, Legato and IBM) have worked furiously to offer users new versions of their respective products to support the trend.

As well as promising to save data to disk, in some cases, their software also allows emulation of virtual tape libraries to disk.

Analysts maintain that backup-to-disk is nothing more than the next step in the evolution of a business storage architecture. Backup-to-disk is a gift for hardware vendors desperate to energise the stagnant IT buying cycle.

Tape backup
There is no question that disk-based backup platforms are rising in popularity. The industry is also moving away from hybrid solutions with staging on disk and final archiving on tape.

But tape continues to play a key role for a number of reasons. The most obvious is financial. Not every business has the money or the inclination to make a wholesale switch to disk.

And no one rushes into technology upgrades for the sake of it. Performance improvements need to be weighed against total cost of ownership.

Tape libraries also have big advantages. They are a good fit for those offsite disaster recovery sites and offer almost limitless capacity.

And, perhaps most importantly, tape is something the business understands and has considerable experience of handling. An effective disk backup strategy requires a degree of re-education.

So, while disk is the future, not everyone is in a big hurry to embrace it.

Disk versus tape: the plus points

DISK
High-speed backup and recovery cuts the time spent cleaning files.

Quick access to information.

Disks are replaceable.

Raid functionality boosts security levels.

TAPE
Business have plenty of experience of handling tape.

Offers almost limitless capacity.

Fits well with offsite disaster recovery sites.

It would be expensive to switch to disk.