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[Xen-users] NPIV & multipath inside domU

Hopefully someone can help get multipath working inside a domU.  I'm using Xen 
4.2.3 on a SLES 11 dom0, and both a SLES11 domU and a CentOS 6 domU.  I'm using 
NPIV to connect my SAN disks to my domU, and, due to the way my SAN presents 
the LUNs, need to use multipath inside the domU.  I've tried this on both 
SLES11 and CentOS6 with pretty much the same result.  The root issue seems to 
be that no "disk identity" (SCSI ID/WWID/etc.) is passed through from dom0 to 
domU on the NPIV connection (or any other xenblk disk connection).  The symptom 
is that scsi_id exits with an error (1) and so multipath, in its default 
configuration, won't create the maps for the multipath devices.

So, I have a couple of ideas, wondering if anyone has ever done either of these 

First, perhaps there is a way to manually set up a multipath map, rather than 
having the domU try to detect it?  Has anyone ever done this?  My Google 
searches have turned up very little useful info, here - mostly articles about 
manually failing over/back a path - which is not what I want.  I'm not sure if 
the multipath.conf file supports manual multipath maps, but this would be an 
acceptable work-around for me, since I'm statically defining the disks that are 
being passed through over NPIV, so they're not likely to change much.

Second, and perhaps a little safer, would be to try to create my own multipath 
ID script that could look at something unique about the disks and make a 
determination.  I thought about something like reading in the first X bytes 
(partition table) of the disk and doing a sha1sum on them, since that should be 
fairly unique for each disk.  Does anyone have any ideas on the best way to do 
this?  Maybe the size of the disk, plus the first 512 bytes or something like 
that?  I'm open to suggestions on what will be the best guarantee of uniqueness 
of disks.

And, yes, I know one way to do this is to just present the LUNs to dom0 and 
have dom0 do the multipathing, but, for various reasons, it's better if I can 
figure out a way to use NPIV and accomplish this.


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