However, while an autoloader combines several slots for tapes and a robotic handler with one tape drive, the vendor terminology isn't consistent. Some manufacturers offer devices with one drive, but no room to add more at the tape library. Others, like Sun Microsystems Inc. StorageTek SL24, offer two half-height drives in place of a single full height drive.
As such, an autoloader falls between expensive libraries which have two to dozens of tape drives, and a standalone tape drive which has no robotics to change the tape.
An autoloader is more expensive than a standalone drive and cannot handle large storage volumes or parallel tape access. Prices range from approximately $1,500 to more than $10,000, depending on the number of slots, the tape technology and other similar factors.
Total tape capacity ranges from several gigabytes or so up to multiple terabytes, again depending on slots and technology. Products start at 500 GB or so at the low end, with most higher capacity units topping out at between 20 TB to 40 TB.
The limit of an autoloader results more from the limitations of having one tape drive than the number of slots for tapes.) At the top end, the limit is more the performance of a single tape drive device.
Tape autoloaders are available from most of the major storage vendors, including Dell Inc., Hewlett-Packard (HP) Co., IBM Corp. and Quantum Corp., as well as more specialized storage vendors such as Overland Storage.
When selecting an autoloader, you need to answer a number of questions, such as:
How much data storage do you need now; how much will you need in five years?
As a rule of thumb, you can expect an autoloader to last for approximately five years. Unlike tape libraries, autoloaders often aren't expandable --- although this is changing due to market demand.
Some autoloaders are now expandable to take more slots. For example, the Dell PowerVault can be expanded from eight slots to 16 slots. Others have features which make handling tape collections easier. IBM's TS2900, for example, comes with removable magazines.
Autoloaders vary greatly in capacity, depending on the tape technology chosen and the number of slots available for tape cartridges. Likewise, they vary considerably in price.
The question of capacity divides into two areas. One is the maximum number of tapes the autoloader can handle. The other area is its throughput at maximum capacity, which relates to staying within your backup window.
What's your backup window; is it likely to shrink over the next few years and if so, by how much?
If an autoloader can't back up your data within the allotted time, either now or in the future, you have a problem.
Tape capacity, speed, etc. are questions that relate to the tape technology, not the autoloader. Most popular drives, such as DLT and various generations of LTO, are available in autoloaders, although most only support two or three technologies.
How many tapes?
Autoloaders typically can handle anywhere from two to dozens of tapes.
Since typically there is only one drive, adding more slots for tapes doesn't increase throughput and may slow it down by increasing the load and unload times. However, if you have a lot of tapes which are infrequently accessed, it may make sense to use an autoloader with a lot of slots.
What is the mean time between failures (MTBF) for the tape drive and the tape handling mechanism?
The MTBF for the drive will be determined by the drive manufacturer's specification. All autoloaders using the same drive will have the same MTBF. However, the tape handling mechanisms are not standardized and the failure rate (usually given in load-unload cycles) may be quite a bit different.
What tape technologies are available?
Most autoloaders can support several different tape technologies, with DLT and LTO being the most popular (although not at once). You need to decide which kind of tape you want to use.
You also must know if the autoloader you're considering is supported by your backup software. Generally speaking, native support for the autoloader by your software is the best option, with emulation as a second option.
Depending on capacity needs, an autoloader can be an excellent choice for SMBs. They are cheaper than libraries, and more convenient and less error-prone than standalone tape drives. However, when considering an autoloader versus a library, you need to match throughput against your backup window.
About the author: Rick Cook specializes in writing about issues related to storage and storage management.
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This was first published in May 2009