Tape Drive/Technology Comparisons (AIT Vs LTO Vs DLT Vs SAIT)

Lets first of all compare the drive characteristics of Current Generations:

UPDATED WITH LTO-4

Recording methods

Linear Serpentine Recording
All DLT and LTO tape products write linear serpentine data tracks parallel to the edge of the tape (Figure 1). In these technologies, a half-inch tape moves linearly past a head assembly that houses the carefully aligned read and write heads. To create the serpentine pattern on the tape, the head assembly moves up or down to precise positions at the ends of the tape.

Helical Scan
Sony AIT employ a helical scan recording method in which data tracks are written at an angle with respect to the edge of an 8 mm tape. This is achieved by wrapping magnetic tape partially around an angled, rotating drum. The read and write heads are precisely aligned in the drum and protrude very slightly from its smooth outer surface. As the tape is moved past the rotating drum, the heads create an angled data track on the tape (Figure 3).

Tape Loading and Cartridge Handling
In all tape drive systems, the tape must be pulled from the cartridge, guided through the tape path, and then pulled across the read-write head assembly. Linear and helical tape technologies differ significantly in their methods of tape handling and loading, but in every case, tapes must be handled properly to avoid high error rates, tape damage, and—in the worst case—loss of data.
Linear Drive Mechanisms

When the tape cartridge is inserted into a linear tape drive, a load mechanism inside the drive engages with a positioning tab at the beginning of the tape, which pulls the tape out of the cartridge and onto a take-up hub inside the drive compartment. As the read or write operation is performed, the tape is spooled between the take-up hub inside the drive and the cartridge supply reel inside the media cartridge. This is one reason why linear tape drives are much larger than helical scan drives, which employ a dual-spool cartridge design. It is very important that linear tape cartridges not be dropped or roughly handled because the tape inside may slacken or shift on the spool. This may cause problems with loading the tape or may cause edge damage on the media, since the leader may fail to engage when inserted into the tape drive. If this leader-latching problem occurs, the tape cartridge is typically rendered useless, and the drive may even require repair, which is particularly problematic in automated tape library environments.

Helical-Scan Drive Mechanisms
Sony AIT drives employ a more common method of tape loading. When the tape cartridge is inserted, drive motors engage the cartridge hubs and work with tape loading guides to position tape into the tape path. As the read or write operation is performed, the tape is spooled from one cartridge hub to the other. Because of this, Sony AIT tape cartridges are much less sensitive to rough handling and dropping. For best results, users should follow the manufacturer’s recommendations for storage and handling of data cartridges.

Tape Tension and Speed Control
In all tape drives, the tape must be precisely moved through the tape path and across the heads during read or write operations. Also, the relative speed between the tape and the heads must be precisely controlled. AIT tape drives employ traditional servo-driven capstan-and-pinch-roller designs to control tape speed. These designs use a capstan, or a controlled motorized cylinder, to pinch the tape against a freewheeling roller, pulling the tape through the tape path at a regulated speed.

The take-up and supply hubs are used to spool and unwind the tape, but the precise tape speed is controlled at the capstan point. The speed of the hubs is engineered to be constantly and precisely varied as the diameter of the two spools changes. For instance, the take-up hub speed must decrease steadily as the tape spool gets larger in order to maintain a constant tape speed across the heads. The goal of the capstan-less design is to reduce tape stress caused by the capstan-and-pinch-roller system.

Tape Speed and Stress
Linear drives move tape at a relatively fast rate, typically over 150 inches per second (ips). The helical scan drives use a much slower tape speed of less than one ips through the tape path and past the rapidly rotating drum assembly. Interestingly, the relative tape speed is nearly equal in both helical-scan and linear technologies.

Tape stress is a function of many system variables, some of which include tape speed, tape path control mechanisms (usually guide rollers), capstan pressure, and media contamination. It is important to understand how each drive technology minimizes this tape stress. Linear tape drives utilise a straighter tape path but a much higher tape speed, making the guide-roller system critical to minimize edge wear on the media. On the other hand, helical scan drives use a much slower tape speed but a more complex tape path.

Data Streaming and Start/Stop Motion
A tape drive’s ability to continuously read or write data, or “stream” data, is a key performance and reliability differentiator. A drive’s performance will suffer dramatically if the drive is not supplied with data at a rate sufficient to keep it streaming. In cases where these conditions are not met, the drive will need to stop the forward tape motion, reverse the position, bring the tape back to speed, and then restart the write operation.

Linear technologies, with higher tape speeds, do not operate well in start-stop mode. Each start-stop operation requires the mechanism to stop the tape from greater than 150 ips, rewind well past the last data written, ramp the speed back to greater than 150 ips, and then resume writing. The amount of time spent performing a stop-rewind-start motion dramatically impacts the overall tape system’s throughput. In an attempt to minimize this, high-performance linear technologies employ powerful reel motor systems. The reel-motor system results in linear drives having larger physical footprints and higher power consumption ratings than helical-scan devices.

Helical-scan drives, in addition to being smaller and using less energy, can perform the stop-rewind-start sequences very quickly. This is owing to their slower tape speeds and their constantly rotating drum mechanisms. While continued stop-start motion is detrimental to any drive, the reliability impact is greater on devices with higher tape speeds because of the mechanical stress placed on the system and the media.

All four of these tape drive technologies use data buffering techniques to minimize the need to perform stop-start activities. Linear technologies must use larger buffers since the performance and reliability penalty for a stop-start operation is so much higher than with helical-scan products. Mammoth and AIT drives will typically out-perform DLT and LTO drives in applications where drive
streaming is not possible.

More on Tape Drive Technologies to come..... Stay tuned

Thanks goes to: SpectraLogic, SearchStorage.com, howstuffworks.com, technologyinreview.com, usenix.net, supercomp.org, copansys.com, for providing the necessary information to publish this information