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From: "Guillaume Lacôte" <Guillaume@Lacote.name>
To: linux-kernel@vger.kernel.org
Subject: BIO ordering and NativeCommandQueueing
Date: Tue, 15 Jun 2004 12:02:12 +0200	[thread overview]
Message-ID: <200406151202.12884.Guillaume@Lacote.name> (raw)

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 ? 

             reply	other threads:[~2004-06-15 10:02 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2004-06-15 10:02 Guillaume Lacôte [this message]
2004-06-15 11:36 ` Jens Axboe

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