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From: David Gilbert <dgilbert@dclg.ca>
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Date: Tue, 12 Oct 2004 11:33:15 -0400
To: "Kenneth D. Merry" <ken@freebsd.org>
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Subject: Re: sata raid & write cache state
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>>>>> "Kenneth" == Kenneth D Merry <ken@freebsd.org> writes:

Kenneth> See my previous mail.  SATA disks differ in two ways:

Kenneth> 1.  Many don't support tagged queueing.

I'd like to see more information on this.  I was under the impression
that SATA required some form of command queueing in all drives.

Kenneth> 2.  If the SATA disk does support tagged queueing, there is
Kenneth> still a fundamental problem with the queueing model in SATA
Kenneth> (and probably ATA, not sure).  According to a coworker of
Kenneth> mine (hardware engineer) who is a SATA expert, the status
Kenneth> phase on the bus is the same phase as the data phase.  So you
Kenneth> basically have to send all the data to the drive on a write
Kenneth> and the drive has to send the status back before the drive
Kenneth> can accept any more data for another queued write command.
Kenneth> So that limits you, effectively, to writing data for one
Kenneth> command at a time.

It would appear that the SATA folks are introducing 'NCQ' (Native
Command Queueing) ... which does supply out-of-order returns among
other things.

Dave.

-- 
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|David Gilbert, Independent Contractor.       | Two things can only be     |
|Mail:       dave@daveg.ca                    |  equal if and only if they |
|http://daveg.ca                              |   are precisely opposite.  |
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