Tandberg SLR: a superior alternative to DDS - DDS Replacement

Computer Technology Review, August, 2002 by Willy A. Johansen

Choosing the right backup technology is key in securing business-critical data. Growing capacity requirements, the need for shorter backup windows and increased security threats all make the decision on a tape backup device more complicated than ever. This article will provide you with the basic information you need to make your decision a little bit easier.

There are two main technologies of tape drives for backup: Helican Scan and Linear Recording.

Helical Scan Products: Track is recorded diagonally in "stripes" across face of longitudinal tape. All current helical scan products utilize a dual hub driven cassette/cartridge and a rotating multi-channel recording head. A separate motor is required to control each tape hub as well as one for spinning the recording head (three electric motors).

In addition the tape guidance path on all helical drives is contained fully inside the drive (requiring precision mechanical components/guides) adding complexity/cost. Generally two other actuator motors are required for cartridge housing load/unload and tape threading/wrap around the head.

As far as capacity futures are concerned DAT/DDS, 8mm, AIT, and VXA helical scan products have less available total tape area than the linear technology products Travan, SC50, SLR, LTO, and DLT tape formats. While the smaller cartridge size is "sexy" it is a liability when it comes to ultimate data storage capacity.

Linear Recording Products: Tracks are recorded parallel with face of longtitudial tape. In linear drives the basic recording technique is similar between Travan, SLR, SC50, VS, DLT, and LTO drives (there are variations in recording density/encoding, track pitch, and number of parallel channels). However there are differences in how the tape is handled between the single reel cartridge based drives and the dual reel cartridge based drives.

The larger recording surface provided by linear technology drives allows for greater storage capacities in linear products at equivalent data packing densities. Linear recording technique also allows the scaling of data transfer rate, adding more recording channels in parallel. In contrast, It is more difficult to substantially increase the transfer rate on helical formats by addition of parallel channels. Limitations come in to play related to relative head to tape speed with helical scan drives where both the tape and the head are moving.

Within the linear recording technology, SLR tape drives utilize cartridges that contain both tape guidance components and a portion of the tape drive system. This makes for a more complex cartridge, but the benefit is the tape drive mechanics required to make use of the cartridge are simplified. SLR tape drives only require one motor to drive the tape, one to two actuators to move the head, and one motor to load/unload the tape. An added benefit of this system is less probability of having interchange problems between different drives. In other drive styles, which fully contain the guidance mechanics in the drive, it is possible to have minor variations in track placement between drives (leading to data interchange problems).

All DLT and LTO style drives require a take up reel located in the drive, so these drives require two drive motors for tape motion. In addition the tape cartridge contains no tape guidance facility thus the drive must contain the precision tape path. These characteristics result in additional cost and also means the tape/drive system is more susceptible to the interchange errors mentioned previously.

Cost of Ownership

Mainstream hard disk drive capacity is moving from 18GB to 36GB and soon to 73GB. Tape backup and restore must grow with these capacities. The need to have all backup data on a single tape cartridge, particularly during a restore process. is critical to increased security and easier administration. Smaller backup windows necessitate faster transfer rates as well. The capacity requirements and the backup/restore times will be important factors to consider in the total cost of ownership.

Price at the time of purchase is often a critical factor. However if one looks at cost of ownership over time (e.g. three years) the picture may change both for the customer and the seller. For a backup system one has to take into account the reliability of the tape drive, and reliability and durability of the media. Annual failure rate (AFR) tells something about how many tape drives of a population may have to go to service per year, and also what the chance is for failure during backup and/or restore. There are also big differences in durability of the media cartridge between the different technologies. Exchanging the tape cartridge every year vs. changing cartridges only every third year has a big impact on overall cost of ownership. The durability of the tape cartridge is not only caused by number of passes of the media over the head, but also by the ruggedness of the cartridge itself.

Where the cost and reliability differences between the technologies come into play are in the components used directly for the recording process such as the magnetic heads, electric motors required, actuators required, and precision mechanical components to implement the recording format. In these areas the technologies vary widely. Less parts typically mean higher reliability. The head design is a big contributor to reliability. Reliability also has to do with how well the microcode (or firmware) for controlling the hardware and host interface is tested and proven.