Editing Tape drive (section)

=== Technical limitations ===
[[Image:LCM - CDC 606 Tape Transport 01.jpg|thumb|[[Control Data Corporation]] 606 tape drive, showing two long vertical vacuum columns in the lower part.|alt=A large cabinet, about the size of an upright refrigerator, with a glass-covered top part holding two reels of magnetic tape, and a bottom part with control buttons framed by vertical channels.]]
A disadvantageous effect termed '''{{visible anchor|shoe-shining}}''' occurs during read/write if the data transfer rate falls below the minimum threshold at which the tape drive heads were designed to transfer data to or from a continuously running tape. In this situation, the modern fast-running tape drive is unable to stop the tape instantly. Instead, the drive must decelerate and stop the tape, rewind it a short distance, restart it, position back to the point at which streaming stopped and then resume the operation. If the condition repeats, the resulting back-and-forth tape motion resembles that of [[Shoeshiner|shining shoes with a cloth]]. Shoe-shining decreases the attainable data transfer rate, drive and tape life, and tape capacity.

In early tape drives, non-continuous data transfer was normal and unavoidable. Computer processing power and available memory were usually insufficient to provide a constant stream, so tape drives were typically designed for ''start-stop'' operation. Early drives used very large spools, which necessarily had high inertia and did not start and stop moving easily. To provide high start, stop and seek performance, several feet of loose tape was played out and pulled by a suction fan down into two deep open channels on either side of the [[tape transport|tape head and capstans]]. The long thin loops of tape hanging in these ''[[Vacuum column (tape drive)|vacuum columns]]'' had far less inertia than the two reels and could be rapidly started, stopped and repositioned. The large reels would move as required to keep the slack tape in the vacuum columns.

Later, most tape drives of the 1980s introduced the use of an [[buffer (computer science)|internal data buffer]] to somewhat reduce start-stop situations.{{efn|Some modern designs are still developed to operate in a non-linear fashion. IBM's 3xxx formats are designed to keep the tape moving irrespective of the data buffer—segments are written when data is available, but gaps are written when buffers run empty. When the drive detects an idle period, it re-reads the fragmented segments into a buffer and writes them back over the fragmented sections—a 'virtual backhitch'.<ref name=ibm3xxxref1>{{cite web
 | url = http://news.techworld.com/storage/3238/mainframe-tape-lock-in-ended/
 | title = Mainframe tape lock-in ended
 | last = Mellor |first = Chris
 | date = 2005-03-02
 | work = TechWorld
 | url-status = dead
 | archive-url = https://web.archive.org/web/20120605022216/http://news.techworld.com/storage/3238/mainframe-tape-lock-in-ended/
 | archive-date = 2012-06-05
}}</ref>}} These drives are often referred to as ''tape streamers''. The tape was stopped only when [[buffer underrun|the buffer contained no data to be written]], or when it was full of data during reading. As faster tape drives became available, despite being buffered, the drives started to suffer from the shoe-shining sequence of stop, rewind, start.

Some newer drives have several speeds and implement algorithms that dynamically match the tape speed level to the computer's data rate. Example speed levels could be 50 percent, 75 percent and 100 percent of full speed. A computer that streams data slower than the lowest speed level (e.g., at 49 percent) will still cause shoe-shining.