Re: SCSI device rescan, detection of disconnected device, or switched devices.

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Thu, 2008-07-31 at 19:09 +0300, Gal Rosen wrote:
> On Thu, 2008-07-31 at 09:15 -0500, James Bottomley wrote:
> > Oh ... you're not really talking about hotplug, which is why everyone is
> > confused.  Hotplug is when you add or remove something from the bus.
> > What you've done is reconfigure the array.
> > 
> > Most of the hotplug we do depends on the transport model (because what's
> > on the transport is changing).  Array reconfiguration has no hotplug
> > event because SAM-3 has no real way of passing the information
> > asynchronously.  The best it can do is Unit Attention/reported luns data
> > has changed (asc=0x3f/ascq=0xe) on the next command.
> > 
> > The problem is that there's no way to process the event correctly even
> > when we get it.  All we can do is issue another report LUNS command and
> > compare.  However, just because it looks like a single LUN disappeared
> > doesn't mean the others weren't permuted or altered in some way (which
> > data we cannot get).
> > 
> 
> Why? if I keep a mapping between sg devices and device id (Inquiry page
> 0x83) then I have all the data.

No we don't, it depends on array policy.  Most arrays when told to take
10% of space from 10 luns and create a new one won't change the WWN of
the existing 10 LUNS, so we have no idea what just happened or what we
should do about it (incidentally, this is an example of a
reconfiguration you probably wanted to do offline anyway).

James


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