On Wed, 7 Jan 2009, T. Horsnell wrote:

> It seemed to me that hardware compression could result in the tapedrive
> mechanism not being fed data fast enough to keep it streaming at full speed,
> since the records may be shortened by the compression process,  whereupon
> it would slow down a bit by bit in order to avoid stopping altogether
> (a really bad thing which results in shoe-shining motion) until the data
> rate increased again, at which point it would accelerate.

LTO drives will operate at reduced tape speeds if there's not enough data
to run at full speed, specifically to avoid shoeshining.

The inmportant thing is to ensure the spool disk array (and it must be an
array, or ramdisk, etc) is fast enough to keep the LTO input saturated -
and that means an ability to sustain streaming throughput significantly
in excess of the LTO native speed.

This in turn limits the number of simultaneous backup/restore jobs we can
perform per LTO drive/array combination. As LTOs get faster, the spool
disk hardware must become dedicated per tape drive.

There are a bunch of tradeoffs involved, but at the end of the day these
two hardware axioms are as true as ever:

1: "Speed costs money. How fast do you want to go?"

2: "Good, fast, cheap -  pick any two"




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