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Arno Lehmann wrote:
  

Hi,

04.09.2008 14:00, T. Horsnell wrote:
    

FMI, will someone please tell me how shoeshining manifests itself?
Does it mean that the tape actually reverses periodically?
      

Yes.

    

e.g. to backspace over a filemark in order to overwrite it, or does it 
mean that the tape just stop-starts rather than continuously streaming. 
If its just stop-start, why does it result in more tape wear (as opposed 
to tapedrive wear) than streaming?
      

Because a tape can not be stopped and started in an instant - the tape 
has mass, it moves with high speed, so even if you had a drive 
mechanism that would be capable of starting and stopping 
(near-)instantaneously, the tape would break.

What happens is this:
- Tape is driven at writing speed.
- Drive runs out of data and, after the last byte in its buffer is 
written, it stops the tape. Some tape passes until the tape comes to rest.
- Drive gets more data
- Drive rewinds the tape until safely before the current end-of-data 
on tape
- Drive starts moving the tape forward
- Once read speed is reached, the drive syncronizes to the tape tracks 
and reads until the current end-of-data is reached.
- In this instant it switches to write mode and writes the current data.
Sometimes the first attempt to find the writing position will not 
work, so this process can iterate a few times.
The result is that the tape passes the read/write heads a - sometimes 
huge - number of times. Each pass rubs some matter from the tape and 
the heads.
Furthermore, the strain on the tape itself is highest when accelerated 
and stopped.

Todays tape technology, which writes hundreds of MB in seconds, really 
suffers if you feed bursts of data to it - a GB of data, fed in small 
bursts, can result in dozens of start-stop-cycles.

Another effect is that, when the write position is found, a small 
length of tape can pass unused before actual data is written again, so 
you lose tape capacity. Similarly, a file mark on tape can use a 
considerable length of tape - both effects decrease the useable tape 
capacity.

So you really want to avoid this shoeshining and lots of file marks.
    

Thank you for this excellent description. I learned a little myself.

- --
 ---- _  _ _  _ ___  _  _  _
 |Y#| |  | |\/| |  \ |\ |  | |Ryan Novosielski - Systems Programmer II
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