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This is why I said that you'll almost always be shoeshining without
spooling. Many modern drives are so fast that you theoretically need to
have perfect conditions to avoid shoe-shining. Though, I suppose if
you're streaming a LOT of data, you're OK.

I wonder how many drives do variable speed writing.

John Huttley wrote:
> To follow up, LTO drives are very fast and this can be a liability.
> However some drives, notably those by HP and some models by Tandberg support
> variable speed writing. Normally down to 50% of normal.
> These are resistant to shoe shining even without spooling.
> 
> --john
> 
> Ryan Novosielski wrote:
> 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.
> 

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