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D2D2T a nice compromise in disk vs. tape debate

By  Rick Cook

What you will learn from this tip: Tape is no longer synonymous with backup, but don't count it out just yet. Many of the current disk-based backup offerings still rely heavily on the old standby.


It used to be that backup automatically equaled tape. But with the falling price of disk drives, new disk-based products and shrinking backup windows, the choice isn't automatic anymore.

Tape still has a cost advantage over disk. Even though disk prices per GB have been falling much faster than tape, tape is still cheaper on the whole. Tape has secondary advantages such as portability and high capacity. It's also a well-proven technology, and that gives it a high comfort factor. In every other respect, the advantage lies with disk. Disks are much faster and it's a lot easier to access and manage data stored on disks.

Despite vendors urging disk-only solutions, the trend isn't so much to replace tape with disk as to supplement existing tape systems with disks that serve as partial backup. In some cases, this is a true disk-to-disk-to-tape system where the data is transferred to disk and immediately written to tape. In other cases, the disks handle the immediate backup and data is written to tape for long-term backup or archival storage.

Factoring in archival storage is important, because backup and archiving are not the same thing. The assumption in backup is that the data will only be maintained for a few days to a couple of months, and that therefore administrators will be dealing with a finite, manageable quantity of data. In archival storage, where data is kept for years, tape is still the superior solution, because it is cheaper, more durable over the long term and more portable.

However, most enterprise data doesn't need to be archived. You need to archive material for several purposes: Regulatory or corporate policy requirements, data that doesn't lose value over time (such as an oil company's geological survey results) and data that is important historically or because it can be repurposed by something such as data mining. It isn't unusual to need to save less than 10% of a company's information for more than three months. By storing the data temporarily on disks, especially inexpensive iSCSI SATA disks, and only archiving data as necessary, companies can keep from outgrowing their existing tape libraries and drives.

In the end, the choice of tape, disk or disk-and-tape needs to be driven by your actual requirements and economic constraints. Don't assume that one class of solutions is right for you.

For more information:

Tip: When to choose tape

Tip: Pros and cons of disk-to-disk backup

Topics: Tape backup

About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

29 Mar 2005

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