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On Mon, 25 Nov 2002 00:54, Donovan Baarda wrote:
> I'm pretty sure most device drivers for both IDE and SCSI do some degree of
> command-reordering before issuing the commands down the buss. I wonder how
> much real-world benefit can be gained from drive-level command re-ordering,
> and how many SCSI drives actualy bother to implement it well :-)

Last I heard was that they both did it badly.  Commands were re-ordered at the 
block device level (re-ordering commands sent to a RAID device is not 

This is separate to re-ordering within the disk.

> The point is, 4 IDE buses will probably match 1 SCSI bus for sustained
> transfer rates....4x133 =533MB/sec... more than 1x the fastest SCSI. Throw
> in the IDE crappy performance, and you get.... about the same.

To sustain that speed you need 66MHz 64bit PCI, which almost no-one gets.

If you have a single 33MHz card then the entire bus runs at 33MHz, so 
therefore you need an expensive motherboard with multiple PCI buses and RAID 
controller cards to support it.

Running two hardware RAID cards on separate PCI buses and then doing software 
RAID-0 across them to solve PCI bottlenecks is apparently not that uncommon.

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