In tape backup, shoeshining (also known as backhitching) is the repeated back and forth motion a tape drive makes when there is interruption in the data stream.
When data is sent to tape, it needs to flow at the same speed the tape is running. If there is a mismatch between speeds, the tape drive will try to accommodate it. For instance, if a backup server is sending data slower than the tape drive processor writes it, the drive will periodically stop and wait for data to catch up. Once the drive determines there is enough data to start writing again, it will rewind to the exact place where the last write took place, reset the tape heads and continue. All this happens very quickly; to the casual observer, the wheels on the tape drive just seem to stop and jerk back and forth (like someone shining their shoes) before moving forward smoothly again.
Because tape is a medium that requires blocks of information to be recorded or read sequentially, blank spaces are not acceptable. Buffers (temporary storage areas) can help with temporary speed mismatches, but they can fill up or empty when the mismatch is chronic.
Excessive shoeshining indicates there is a problem. Repeated back and forth motion not only causes a degradation of service, it also causes excessive wear and tear on both the tape and the tape drive. To prevent shoe-shining, backup administrators need to strike the right balance between the number and speed of backup tape drives, the amount of data being sent to the tape library and the connectivity between components.
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