BIO ordering and NativeCommandQueueing

[Guillaume Lacôte]

Hello,
(I hope this is the right place for this - sorry if it is not).

Native Command Queueing (and Tagged Command Queueing) is a feature provided by 
the hardware of newer IDE (and old SCSI) disk drives which basically consists 
in reordering the commands issued on the ATA bus to improve speed.

I assume however that the fastest way to read sectors 101 to 110 is to ask for 
them in that order: 101,102,...,110 . This is a basic assumption made by most 
OSes and apps I presume (otherwise for example DMA performance would be 
catastrophic).

Here is my point: since a bvec consists of _ordered_ requests only, what is 
the use of NCQ ? Requests will arrive to the drive in increasing order, which 
is the best possible ordering performance-wise; thus NCQ will do never do 
anything.

Am I mistaken ? 
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