12.16 Tape Drives


12.16 Tape Drives

Tape drives are also popular mass storage devices. Traditionally, personal computer owners have used tape drives to back up data stored on hard disk drives. For many years , tape storage was far more cost-effective than hard disk storage on a cost-per-megabyte basis. Indeed, at one time there was an order of magnitude difference in cost per megabyte between tape storage and magnetic disk storage. And because tape drives held more data than most hard disk drives, they were more space-efficient too.

However, because of competition and technological advances in the hard disk drive marketplace , tapes have lost these advantages. Hard disk drives are now exceeding 250 GB in storage, and the optimum price point for hard disks is about $0.50 per gigabyte. Tape storage today costs far more per megabyte than hard disk storage. Plus, only a few tape technologies allow one to store 250 GB on a single tape, and those that do (such as Digital Linear Tape, or DLT) are extremely expensive. It's not surprising that tape drives are seeing less and less use these days in home PCs and are typically found only in larger file server machines.

Back in the days of mainframes, application programs interacted with tape drives in much the same way that today's applications interact with hard disk drives. A tape drive, however, is not an efficient random access device. That is, although software can read a random set of blocks from a tape, it cannot do so with acceptable performance. Of course, in the days when most applications ran on mainframes, applications generally were not interactive, and the CPUs were much slower. As such, the standard for 'acceptable performance' was different.

In a tape drive, the read/write head is fixed, and the tape transport mechanism moves the tape past the read/write head linearly, from the beginning of the tape to the end of the tape, or vice versa. If the beginning of the tape is currently positioned over the read/write head and you want to read data at the end of the tape, you have to move the entire tape past the read/write head to get to the desired data. This can be very slow, requiring tens or even hundreds of seconds, depending on the length and format of the tape. Compare this with the tens of milliseconds it takes to reposition a hard disk's read/write head. Therefore, to perform well on a tape drive, software has to be written with special awareness of the limitations of a sequential access device. In particular, data should be read or written sequentially on a tape to produce the highest performance.

Originally, data was written to tapes in blocks (much like sectors on a hard disk), and the drives were designed to allow quasi-random access to the tape's blocks. If you've ever seen an old science fiction movie with the old-style reel-to-reel drives, with the reels constantly stopping, starting, stopping, reversing, stopping, and continuing, you were seeing 'random access' in action. Such tape drives were very expensive because they required powerful motors, finely tooled tape- path mechanisms, and so on. As hard drives became larger and less expensive, applications stopped using tape as a data manipulation medium and used tape only for offline storage. Using a tape drive was simply too slow for normal application work. As a result, most systems started using tape drives only in sequential mode for backing up data from hard disks.

Because sequential data access on tape does not require the heavy-duty mechanics of the original tape drives, the tape drive manufactures sought to make a lower-cost product suitable for sequential access only. Their solution was the streaming tape drive, which was designed to keep the data constantly moving from the CPU to the tape, or vice versa. For example, while backing up the data from a hard disk to tape, a streaming tape drive treats the data like a video or audio recording and just lets the tape run, constantly writing the data from the hard disk to the tape. Because of the way streaming tape drives work, very few applications deal directly with the tape unit. Today, it's very rare for anything other than a tape backup utility program, run by the system administrator, to access the tape hardware.




Write Great Code. Understanding the Machine, Vol. 1
The Art of Assembly Language
ISBN: 1593270038
EAN: 2147483647
Year: 2003
Pages: 144
Authors: Randall Hyde

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net