linux-kernel - Re: [PATCH] scsi_execute_async() should add to the tail of the queue

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Message-ID: <20061219184453.GD5010@kernel.dk>
Date:	Tue, 19 Dec 2006 19:44:54 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jon Escombe <lists@...sco.co.uk>
Cc:	Arjan van de Ven <arjan@...rus.demon.nl>,
	Dan Aloni <da-x@...atomic.org>,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	linux-scsi@...r.kernel.org, Mike Christie <michaelc@...wisc.edu>,
	Elias Oltmanns <eo@...ensachen.de>
Subject: Re: [PATCH] scsi_execute_async() should add to the tail of the  queue

On Tue, Dec 19 2006, Jon Escombe wrote:
> Jens Axboe wrote:
> > On Tue, Dec 19 2006, Arjan van de Ven wrote:
> >> On Tue, 2006-12-19 at 10:35 +0200, Dan Aloni wrote:
> >>> Hello,
> >>>
> >>> scsi_execute_async() has replaced scsi_do_req() a few versions ago,
> >>> but it also incurred a change of behavior. I noticed that over-queuing
> >>> a SCSI device using that function causes I/Os to be starved from
> >>> low-level queuing for no justified reason.
> >>>
> >>> I think it makes much more sense to perserve the original behaviour
> >>> of scsi_do_req() and add the request to the tail of the queue.
> >> Hi,
> >>
> >> some things should really be added to the head of the queue, like
> >> maintenance requests and error handling requests. Are you sure this is
> >> the right change? At least I'd expect 2 apis, one for a head and one for
> >> a "normal" queueing...
> >
> > It does sounds broken - head insertion should only be used for careful
> > internal commands, not be the default way user issued commands. Looking
> > at the current users, the patch makes sense to me.
> >
> 
> It's worth noting that the hdaps disk protection patches rely on the 
> current behaviour to add 'IDLE IMMEDIATE WITH UNLOAD' commands to the 
> head of the queue.. Another function, or a new parameter for queue 
> position would be needed to retain this functionality - any preference 
> for either?

The hdaps disk protection should not be using the SCSI internal
function, so it should not be an issue. The block layer API exposes both
front/back/sort insertion possibilities.

-- 
Jens Axboe

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