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What's better for backup: tape or disk?

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Incorporating such a device into a backup environment would most likely require the use of traditional backup software, or as in the case of Nexsan, vendor-supplied software, with the disk system configured as the target device. Off-site copies would be handled by either device-to-device replication or by using the backup application to make tape copies.


Disk backup fuels dramatic capacity increase
PRIMARY
STORAGE (GB)
DISK-BASED
BACKUP (DBB)
TO PRIMARY
STORAGE RATIO
ANNUAL
STORAGE
GROWTH
RATE (%)

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YEAR 1
REQUIRED DISK
STORAGE (GB)
YEAR 3
REQUIRED DISK
STORAGE (GB)
  PRIMARY DBB PRIMARY DBB
1000 10:1 50% 1500 15000 3375 33750
1000 5:1 50% 1500 7500 3375 16875
1000 1.2:1 50% 1500 1800 3375 4050

A virtual tape system (VTS) is perhaps the simplest to incorporate into an existing environment. Because it emulates a tape library, it should be no more difficult that adding a new tape device. The functionality of the VTS is highly dependent on the backup software being used. You would still require traditional tape devices to create off-site volumes. Also, it must be pointed out that the VTS systems currently available for open systems environments haven't yet reached the functionality and maturity of those found in the mainframe world.

One concern with both low-cost disk systems and VTS systems is the amount of disk needed to support an environment (see "Disk backup fuels dramatic capacity increase," this page). While this can be partially offset by a reduction in tape media purchases and storage costs, it still appears that tape continues to hold a cost advantage over traditional disk-based solutions.

A third approach to low-cost disk storage is content addressable storage technology. These systems typically consist of a large number of low-profile servers each with two to four ATA disks. They have several particularly interesting features:

  • The servers are clustered in a Redundant Array of Independent Nodes (RAIN). Usually many servers fit in a single cabinet. Like RAID arrays, RAIN protects against failure through redundancy.
  • Data that's written to these devices is cataloged by content, using a hashing algorithm based on the data itself. Therefore, when a piece of data is received that is the same as one that has been previously cataloged, there's no need to write another copy to disk.
  • Systems are self-healing-when one node or disk fails, data is automatically replicated to another healthy node.
  • Replication can be within a single frame or to other local or remote frames providing interesting options for disaster recovery.
  • Because multiple copies of the same data don't need to be stored, the ratio of backup data to primary data in these systems is more like to be approximately two times or less, rather than five to 10 times with standard disk backup.

Tape or disk?
How does one decide if disk makes sense for a particular environment, and if so, which type of disk storage to use? Here are some guidelines:

  • The key advantage to disk is faster restore time.
  • Modern tape devices can backup large files such as databases, as fast as or faster than disk. In environments with many small files, disk should have an advantage.
  • Tape is highly transportable. It's usually easy to send tapes anywhere that they are needed for recovery.
  • Introducing disk devices demands new backup procedures as well as likely reconfiguration of backup software.
  • Buying and storing more tapes is usually easier than adding disk capacity.
Can you completely eliminate tape from your backup environment? Is disk the future of backup? Disk-based solutions are in their infancy. Products will continue to mature, and more new technologies are on the horizon. While tape will continue to advance and play an important role, for many environments, disk will become a major component of backup in the coming years.

This was first published in March 2003

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