Re: /sys/.../enclosure_device:<something> ?

[Anne Mulhern <amulhern@redhat.com>]

----- Original Message -----
> From: "James Bottomley" <James.Bottomley@HansenPartnership.com>
> To: "Anne Mulhern" <amulhern@redhat.com>
> Cc: linux-scsi@vger.kernel.org
> Sent: Wednesday, January 13, 2016 3:30:07 PM
> Subject: Re: /sys/.../enclosure_device:<something> ?
> 
> On Wed, 2016-01-13 at 15:15 -0500, Anne Mulhern wrote:
> > 
> > 
> > 
> > ----- Original Message -----
> > > From: "James Bottomley" <James.Bottomley@HansenPartnership.com>
> > > To: "Anne Mulhern" <amulhern@redhat.com>,
> > > linux-scsi@vger.kernel.org
> > > Sent: Wednesday, January 13, 2016 2:15:05 PM
> > > Subject: Re: /sys/.../enclosure_device:<something> ?
> > > 
> > > On Wed, 2016-01-13 at 11:28 -0500, Anne Mulhern wrote:
> > > > Hi!
> > > > 
> > > > I'm looking for the most precise information available about the
> > > > meaning of <something>
> > > > and from what it is derived. I'm also interested in what the
> > > > values
> > > > in the files in that directory
> > > > may mean. Could somebody point me there?
> > > 
> > > It's in drivers/misc/enclosure.c:enclosure_link_name()
> > > 
> > 
> > Thanks. I will do my best with that.
> > 
> > > > Also, how can there be more than one enclosure_device
> > > > subdirectory
> > > > for the same device,
> > > > or, alternatively, why isn't <something> stored in a file in the
> > > > more
> > > > regularly named directory
> > > > /sys/.../enclosure_device?
> > > 
> > > I'm not sure I parse the question, but if you're asking how can a
> > > single enclosure bay appear to have more than one device, that's
> > > when
> > > there are multiple paths to the device.
> > 
> > I'm afraid the word path is so overloaded...I'm assuming that you
> > are talking about a multipathed environment, where two device nodes,
> > say /dev/sdb, /dev/sdc correspond to the same disk, and are hence
> > multipathed. Of course, in that case, it makes sense that both
> > devices should share a bay. And that's what I observe.
> 
> Yes, that's what I meant.
> 
> > If I understand you correctly, then my question is actually the
> > opposite. Why does it look as if the directories are set up so that
> > there can be multiple bays for a single device...i.e., the directory
> > naming scheme allows me to specify an arbitrary number of bays for
> > /dev/sdb. It seems like there should never be more than one?
> 
> There is only one bay per device since a physical disk can't be in more
> than one bay at once.  Why do you think it was set up to allow >1?
> 

Purely because the naming scheme for the directory,
'enclosure_device:<identifier>' supports more than 1.

This directory pops up in the directory of the parent device representing
a block device, and should represent just the bay for that device, AFAIU.

The most compelling reason for this naming scheme to support more than 1
subdirectory is if it is necessary, i.e., there are more than one possible.

That might be true if my understanding of the block device's parent device's
meaning is incorrect or if something else doesn't mean what I think it means.

- mulhern

> James
> 
> > - mulhern
> > 
> > > James
> > > 
> > > > (It would be nice to be able to obtain the value of <something>
> > > > using
> > > > libudev's attribute
> > > > lookup facilities, but making that value a part of the specially
> > > > named directory enclosure_device:<something>
> > > > makes such a process especially difficult.)
> > > > 
> > > > Thanks,
> > > > 
> > > > - mulhern
> > > > --
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