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Re: ata(4) and NCQ

--- On Tue, 4/19/11, Thor Lancelot Simon <> wrote:

> From: Thor Lancelot Simon <>
> Subject: Re: ata(4) and NCQ
> To: "Jonathan A. Kollasch" <>
> Cc:,
> Date: Tuesday, April 19, 2011, 1:13 PM
> On Tue, Apr 19, 2011 at 07:57:56PM
> +0000, Jonathan A. Kollasch wrote:


> I believe FreeBSD ultimately ended up faking up ATA disks
> to look like
> SCSI ones, and using their existing SCSI midlayer to manage
> tags.

And so, of course, does Linux.

> It would be good to avoid that.  The SCSI code is
> really large.
> However, it does do what's wanted, so perhaps a careful
> examination of
> how it manages tags and openings would be a good first
> step.
> I believe the NCQ rules are much like the SCSI tag ordering
> rules: 

[[ discussion of scsi tags and NCQ]]

Yes, I  believe they NCQ rules are deliberatly like SCSI tagged-queue rules.

> How this maps onto ATA drives and NCQ, I don't know, but
> the overall
> issues and history are worth keeping in mind.

Also that both FreeBSD and Linux took the plunge and handle ATA drives through 
their SCSI mid-layer.  I agree, that code is big; but maybe that alternative is 
worth examiining, too.


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