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SubjectRe: disk head scheduling
Linus Torvalds wrote:
..
> This of it as "scheduling entity". It then depends a lot on the actual
> hardware what such a scheduling entity is.
>
> For example, for floppy drives and IDE disks, you really cannot schedule
> across the controller, because while there are multiple devices per
> controller, they are not independent of each other.

Actually, sometimes we can (and do) schedule multiple devices
for overlapped I/O on the same controller. ATAPI devices (cdrom, tape,
zip/floppy) can overlap some operations with a hard disk (or other ATAPI)
on the same cable/controller.

And I think Gadi added per-device (rather than per-controller) queues
somewhere in the 2.1.xx timeframe, to improve performance of disk<->tape
backups on a single controller.

ATA4/ATA5 now include "queued commands" for even more overlap,
pretty much identical to SCSI. But Linux does not (yet) support that.

Cheers
--
mlord@pobox.com

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