Messages in this thread | | | From | Guillaume Lacôte <> | Subject | BIO ordering and NativeCommandQueueing | Date | Tue, 15 Jun 2004 12:02:12 +0200 |
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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.
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