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Tape and recovery are synonymous. But it's the automation that libraries add to the base technology that make tape a viable road forward for storage managers.
The comparative low cost for backup and recovery ensures tape's continued use to keep up with the explosion of data. New tape technologies need to maintain pace with the increasing time and cost of backup/recovery. Even though redundant and/or remote disk mirroring has become less expensive, and thus more popular, due to its higher speed, tape is still the best solution for many businesses.
Best practices for recovery take a data snapshot from disk and write it to removable tape cartridges. Then the cartridges are sent to a safe, off-site archive. Tape overcomes single points of disk failure like software corruption cascading to the mirror or disasters, including sabotage, affecting both locations. For example, several companies mirrored from one World Trade Center tower to the other before Sept. 11. Others mirrored from their WTC tower to buildings across the street, which were also destroyed.
When using tape, a library is the most efficient method for fast, reliable backup and restoration of large amounts of data. Library automation provides economies of scale that individual drives can't by consolidating data, lowering total cost of ownership (TCO), reducing human intervention and error, simplifying backup and recovery, providing unattended lights-out backup and allowing a scalable growth path. Libraries'
reduced administrative costs have a big impact on TCO. That effect is amplified by using a tape library in conjunction with a storage area network (SAN). But all tape libraries are not created equal, and choosing the right one requires extensive analysis (see "How to choose a tape library").
Tape library configuration
Before you can calculate a tape library's TCO, you need to choose the right combination of tape formats (see "A guide to tape formats") - which determines the capacity and transfer rate of individual drives - and the number of tape slots, which determines the total capacity of your library. The range available to you is quite broad: from autoloaders costing a couple of thousand dollars, to enterprise libraries in the hundreds of thousands of dollars (see "Scoping out tape library vendors").
The first step in automation is an autoloader. An autoloader, by definition, has one drive that typically serves seven or eight cartridges. An autoloader is good for unattended backup for a week or more for smaller companies. The newest versions are rack-mountable in a 2U format and have two drives - in case one fails.
All tape libraries are not created equal, and all their features aren't equally important. Here's what to focus on.
||HOW TO COMPARE
||Total capacity measured in GB/cu ft. = GB/U or ft. For automation use, the higher the density the better.
||Tape drive's transfer rate x number of drives = throughput in GB/hr. Most important to meet recovery time objective, not just backup window. MB/s x 3.2 = GB/hr is ideal throughput take off. Allow 15% for overhead.
||Make sure your tape drives and popular tape drives are supported.
|Maximum cartridge capacity
||More cartridges for more capacity. Each slot holds one tape cartridge.
|Maximum drive capacity
||More drives for faster data transfer. Most start with two or four drives so you can add drives later, when you need faster transfer rates.
|Maximum library capacity
||Tape cartridge capacity x total slots = total GB or TB.
||After meeting above criteria, price is a key differentiator. Make sure and look at total cost of ownership (TCO).
||Varies by vendor from one to three years and on-site to parts exchange.
||Allows monitoring library from any Web-based terminal.
||Native Fibre Channel allows easy SAN attachment and reduces components.
|Average cell to cell
||Speed of robotics affects access time to data, as does cartridge exchange and tape drive load time to data. Faster is better.
|Cubic feet or U
||Physical volume or size of library.
|Cartridge access slot
||Enter cartridges into library without opening door; some use carriers.
|Average power consumption
||Measured in watts. Correlates with ongoing operating cost. Larger libraries are 7-8 times more costly than smaller ones.
||Supports different drive and cartridge types in same library.
||Special location for cleaning tape so you don't have to use access slot.
|Mean time between failure
||Expected reliability measure. All libraries have high ratings and require preventative maintenance.
||Self explanatory, but make sure it fits through door and ceiling.
|Weight of library
||Weight varies significantly, higher densities are sometimes lighter.
|Source: Evaluator Group|
This was first published in June 2002