linux-kernel - Re: [linux-pm] Power management for SCSI

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Message-ID: <48AB55C9.3010106@s5r6.in-berlin.de>
Date:	Wed, 20 Aug 2008 01:22:49 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Oliver Neukum <oneukum@...e.de>, Pavel Machek <pavel@...e.cz>,
	linux-pm@...ts.linux-foundation.org,
	James.Bottomley@...senpartnership.com,
	Linux-pm mailing list <linux-pm@...ts.osdl.org>,
	kernel list <linux-kernel@...r.kernel.org>, teheo@...ell.com
Subject: Re: [linux-pm] Power management for SCSI

Alan Stern wrote:
> On Tue, 19 Aug 2008, Oliver Neukum wrote:
> 
>>> More to the point is whether you should ever suspend any of these 
>>> devices if there can be multiple initiators.  But that's a separate 
>>> question.
>> But one that needs to be addressed.
> 
> One possibility is to have an attribute flag for SCSI transport
> classes, indicating whether the transport supports multiple initiators.
> 
> Besides, isn't this already an issue?  What happens when someone does a 
> system suspend or hibernate?  Don't the attached disk drives get spun 
> down, even if there are other initiators on the same SCSI bus?

In (fw-)sbp2, we have for example this simple code:

static int sbp2_scsi_slave_configure(struct scsi_device *sdev)
{
...
	if (sbp2_param_exclusive_login)
		sdev->manage_start_stop = 1;
...
By setting the exclusive_login module parameter from Y (default) to N, 
multiple initiators per logical unit become possible.  We are too lazy 
to check whether there are actually other initiators at a given moment; 
after all they can come and go all the time.  So the simplest strategy 
is to suppress managed START STOP when concurrent initiators are _possible_.

I suppose though that all multiple initiator capable transports have 
ways to query the presence of other initiators at any given time; but I 
don't think the respective effort is justified.

> (And is this really a problem?  If an error occurs because a drive is 
> spun down when some other device tries to access it, that other device 
> should simply spin the drive back up again.)

The high latency may be a problem.
-- 
Stefan Richter
-=====-==--- =--- =-=--
http://arcgraph.de/sr/
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