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From: Jean-Sebastien ROY <jean-sebastien.roy@wanadoo.fr>
To: freebsd-stable@freebsd.org
Subject: Re: Tagged Queueing and ATA driver (was soft update should be default)
References: <00b601c0d5bd$42ba1340$0901a8c0@haym.transactionsite.com> <Pine.BSF.4.33.0105061425380.84550-100000@merlin.th.physik.uni-bonn.de>
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Date: Sun, 06 May 2001 17:00:39 +0200
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(Jan Conrad) wrote:

> Well, if I read the sources correctly, enabling tagged queueuing in the
> ata driver *IMPLIES* write caching!!!
> 
> So it's pretty clear why tagged queueing is as good as write cacheing (or
> as bad :-)
> 
> sys/dev/ata/ata-disk.c of 4.3R says
> 
>     /* enable write cacheing if allowed and not default on device */
>     if (ata_wc || ata_tags) {
>         if (ata_command(adp->controller, adp->unit, ATA_C_SETFEATURES,
>                         0, 0, 0, 0, ATA_C_F_ENAB_WCACHE, ATA_WAIT_INTR))
>         ata_printf(scp, device, "enabling write cache failed\n");
>     }
> 
> What I really do not understand here - maybe it's trivial, as I am not an
> expert here - is why does tags imply wc for ATA but not for SCSI?
> 
> The SCSI driver says 'Tagged Queueing Enabled' even on my SCSI disk (IBM
> DDYS-T18350N) where WCE is off!
> 
> 
> -Jan

On a related note, is there a way to know if Tagged Queueing and/or 
Write Caching are -really- enabled ?

I've got a Promise controller (<Promise ATA100 controller>), and looking 
at the code, it seems "Tagged Queueing" is disabled on some promise 
controllers. But :
   systctl -a | grep ata
always returns what I've put in /boot/loader.conf ie :
   hw.ata.tags: 1

js

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