Shoeshine problem

The Shoeshine problem is the uneven back and forth movement of a data storage tape when the data feed is slow.

To derive the name

In the United States, shoe shiners offer their services in public places. The characteristic hand movement is the back and forth while polishing the shoes to a shine. This polishing technique ( shoeshine ) gave its name to the start-forward-stop-backward-stop-forward movement when a tape receives data from the server too slowly and therefore has to stop and reposition more often.

Similar phenomenon

This should not be confused with the alternating movement of the two rollers of large magnetic tape machines with vacuum shafts, where the stroboscopic effect can only appear to behave in the same way. Pulling the tape through the second, initially empty roll is not possible with these drives, because the increasing diameter of the tape layers would increase the transport speed at the read / write head during winding , which would lead to a lower data density at the end of the tape. In addition, the physical stress on the tape material and the risk of the tape tearing would be very high. In order to unwind and rewind the tape on the second roll free of stress, the magnetic tape is fed past the read / write head by pressure rollers at a constant speed. In front of and behind are (mostly vertical) vacuum shafts with two light barriers each. The inserted full roll runs until the tape in the first vacuum shaft triggers the lower light barrier, the second roll winds up as soon as the lower light barrier in the second shaft triggers. If the upper barrier is reached there, the second roll stops again. If the upper light barrier is triggered in the first shaft because the tape has been read almost completely from the first shaft, the first roll begins to unwind the tape again.

Streaming problem with tape storage operation

Moving back and forth stresses the belts and the belt mechanism. The ideal is continuous forward movement (so-called streaming). This is why some types of tape devices are also called streamers . Streaming can be supported if a very large buffer memory can bridge the supply gaps. In addition, the belt movement should be slowed down or ramped up gently, similar to a pendulum that slows down and accelerates at the reversal point.

The tape material also plays a role. It should be tear-proof and elastic, without stretching permanently. With a stretched tape, the data bits are no longer in place when the tape is read.

literature

• W. Curtis Preston: Backup and Recovery. O'Reilly Media, Sebastopol 2007, ISBN 0596102461 , pp. 261-262.