why arnt hard heads parralell internally?

[StUdMaN]

some drives have like 6 or 8 heads why dont they gang them up like raid0 interanly?

 

[SCSIRAID]

That would create several problems....

1) Cost... It would require multiple read channels and head preamps

2) Servo issues... Compensation for stack tilt etc...

 

[CTho9305]

it is hard enough to accurately position the heads as a block.... accurate control of each head separately would be obscenely difficult!

A different idea might be to have all the heads read/write at once, to stripe across the platters. I think presently, only one head can be reading/writing at a time, but if there was a way to do it in parallel using all heads at once, you might be able to improve throughput a lot.

 

[alpha88]

Its defiently possible, and probably will happpen in the future. I think it will happen in scsi drives first. All it really needs it to add capability to write/read on all heads at same time + controller to buffer incoming streams -> striped stream and from striped stream to outgoing stream with a VERY low error rate.

 

[StUdMaN]

I didnt mean 8 heads moving independantly-I meant fixed so they move together like stripe a byte instead of just writing a bit

 

[Mark R]

The major problem with this is ensuring that the cylinders on the platters are all aligned. Current methods of manufacture record the cylinders with their spacing very closely fixed, but their actual position on the platter is not fixed as accurately - this means that what may the the optimum position for reading/writing a cylinder on one surface may not be the optimum position on another.

Cylinders are so closely spaced, that relying on any method of finding them (other than locating them with the head that is doing the reading) simply doesn't work with current drive technology. As an example, the IBM Deskstar 120GXP has a minimum cylinder spacing of 0.45 microns (a size on a similar scale to the size of features on a modern CPU).

Even if you could get it to work, you'd have reliability problems due to uneven heating while operating (and therefore uneven thermal expansion causing the heads to go out of alignment) and manufacturing problems such as the spindle or servo not being mounted straight (A misaligment of a few minutes of arc shouldn't be a problem with current drives, but one of these stacked drives wouldn't work at all - even a deviation of a few seconds of arc would cause grave problems).

 

[kpb]

I'm pretty sure seagate introduced one drive that did this at one point. Was the fastest drive around at the time but it was also really expensive and I'd assume very hard to manufacture. It might come up again some time in the future but not untill they have more trouble just upping the rpms.