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What you will learn in this tip: Rather than switching from a tape to a disk-based backup system, why not use a mixture of the two technologies and get the best of both worlds? Learn about the pros and cons of this approach in this data backup management tip.

There's no denying that disk-based backups are all the rage in the past few years, but tape-based backups are still alive and well. As such, organizations may find themselves choosing between disk- and tape-based backups. Rather than choosing, however, it may be more advantageous to use a mixture of the two backup technologies.

To give you a better idea of what I mean, take a look at the chart below "A comparison of tape and disk backup." It compares tape and disk backups in several key areas:

A comparison of tape and disk backup

  Disk Tape
Portability Not usually portable, although it is possible to store backups off-site by backing up to a remote SAN, a cloud application or using RDX removable disk technology. Portable.
Capacity Finite (This is generally only true for a SAN. Every storage array has a maximum [finite] capacity, even if that capacity has not yet been reached.) Each tape has a finite capacity, but backups can span multiple tapes.
Speed Very fast because disks support random access. Somewhat slower than disk due to the linear nature of tapes.
Availability Most recent backups are available. A tape must be loaded before its data can be restored. This can be problematic if the tape is stored off-site.
Reliability Generally reliable, but a single disk error can render an entire series of backups useless because many disk-based backup applications perform block-level incremental backups. Tapes are more reliable than they once were, but are still vulnerable to demagnetizing and to being "eaten" by tape drives.
Administrative burden Multiple versions of files are usually retained online, and users may be able to restore their own files. A user must typically submit a help desk request and then wait for an administrator to restore the backup.
Backup frequency Many disk-based solutions perform backups on a continuous basis throughout the day, ensuring that the latest data is always backed up. Tape-based backup typically occurs late at night. There is a large potential for data loss if a failure occurs before the backup has had a chance to run.
Backup window If the disk-based backup supports continuous data protection (CDP), then the backup window is not an issue. The backup window may not be an issue for any resources that support Volume Shadow Copy (or other snapshot technology) backups, but resources that do not support VSS must be backed up within the allotted window.

So how can you get the best of both worlds? The key is to decide what types of backups are best suited to tape, and which backups would benefit more from disk.

Data backup management strategies: Static and dynamic data

Obviously, every organization has its own unique needs, but generally speaking you should base your data backup management strategy on whether the data that is being backed up tends to be static or dynamic. Disk-based backups are best suited to data that is rapidly changing, but the volume used to store those backups tends to have a limited amount of space. Therefore, if you know that you have certain data that is never going to change then you can free up some space on your disk backups by writing the static data to tape.

This leads me to another point. Some organizations use tape backups to provide a safeguard for disk-based backups. We all know that disks can go bad, or can become corrupted. That is the main reason why we create backups in the first place. So what is stopping the disks in your disk-based backup server from failing? Absolutely nothing.

Some people assume that if the storage array used by a disk-based backup application fails, then nothing has really been lost. After all, the production servers have not failed, and all of the data still exists.

In reality, however, a backup failure is a big deal. For one thing, the failure leaves you with no backups until you can get the problem fixed. The other problem is that your historical backups are lost. Suppose, for example, that a user asks you to restore a file from last Thursday. The file server is still functioning, and the file still exists, but because the backups are gone there is no way to revert to a previous version of the file.

One way around this problem is to back up the backup server. Although you can perform disk-to-disk-to-disk backups, many backup administrators prefer to perform disk-to-disk-to-tape (D2D2T) backups. That way, the backup server has been backed up to removable media. This allows for the creation of backups that can be shipped off-site, and they won't be damaged along with the disk-based backups if the facility were to be hit by lightning.

Both disk and tape are viable backup mediums. Since both mediums have their own strengths and weaknesses, you can create a more robust backup architecture by combining the two mediums.

About the author: Brien M. Posey, MCSE, has previously received Microsoft's MVP award for Exchange Server, Windows Server and Internet Information Server (IIS). Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. You can visit Brien's personal website at

This was last published in October 2010
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