The voodoo of SCSI...

Billy Pettit Billy.Pettit at
Tue Apr 10 11:53:47 CDT 2007

Tom Peters wrote:
SCSI drives often have a spin-up jumper, so that the machine can spin them 
up one at a time to decrease inrush current caused by large arrays spinning 
up all at the same time. Some drives also have a spin-up delay jumper-- 1, 
2, 4 or 8 seconds. Others switch on the motor on first access.
On many drives, the jumper(s) are on the bottom.
I never heard that these lines are brought out to the interface, but also I 
never heard that they're not brought out.
Perhaps that's something to do with it.
Billy wrote:
In the early days of SCSI, spin delay was not standardized.  There were
several versions.  One used a fixed delay, like you mentioned.  But the most
common delayed a fixed number of seconds multiplied by the drive ID number.
So unit 3 would be 3X delay, LUN 4 would be 4X delay, etc.
This was a holdover from the SMD days.  SMD drives would delay until the
unit in front of them on the cable went ready.  So a string of drives would
come up one at time.  The unit number times a fixed delay was an attempt to
speed up the process.  The problem only occurs on the initial inrush to the
motor.  And that current drops rapidly once inertia is overcome.  The
engineers realized that they didn't have to wait for ready, only until the
current dropped to a normal level, usually only 1 or 2 seconds.


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