linux-kernel - Re: deterministic scsi order with async scan

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Date:	Thu, 16 Jul 2009 05:57:03 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	david@...g.hm
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-scsi@...r.kernel.org
Subject: Re: deterministic scsi order with async scan

On Wed, Jul 15, 2009 at 06:09:22PM -0700, david@...g.hm wrote:
> is there any way to get deterministic device ordering with scsi async  
> scanning?
>
> currently (2.6.30) it seems that the various scsi busses are loaded in 
> the order that they are detected, which can vary from boot to boot 
> depending on how long it takes for the card to initialize.

I think you're confused.  The async scsi scanning was designed to _not_
move devices around randomly.  There are other asynchronous schemes in
the kernel, some of which were not designed with the same care.

The SCSI async scan can't do much about it if the busses are detected
in a different order.

> would it be possible to detect the cards/drives, but not register them  
> until all the detection is complete so that they can be registered in a  
> deterministic order?

That's exactly how the scsi async scanning works.

> having two drives on two different controllers swap positions from boot 
> to boot makes it very painful. yes I can make an initrd that fixes this 
> up in user space by examining each drive and creating links to re-order 
> them, but this is a lot of work to fix randomization that can be 
> prevented in the first place.
>
> David Lang
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-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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