software raid auto assembly problem on Xen host

Michael Torrie torriem at
Mon Mar 1 17:31:22 MST 2010

So I have a number of Xen Dom0 hosts that are all connected to a fibre
channel fabric.  Some of my DomU machines employ software RAID across
two physical LUNs brought in from the fibre channel arrays.  The reason
I do this is to get a bit more redundancy.  Should the power fail on one
chassis or the cable gets damaged, the VMs can still function.  Anyway
the problem is that when the Dom0 host boots up, it sees all the volumes
on the fabric (right now some 20 scsi devices in /dev) and then proceeds
to scan and assemble all the RAIDs it finds.  This is exactly what I
don't want to happen as having the Dom0 assemble the RAID and then
booting the DomU which will also assemble same RAID will be very bad indeed.

I've heard that you can label the superblocks on the RAID members with
hostname so they won't auto-assemble except on their proper host.  But
the RAIDs were created by the RHEL installer and I have no idea if they
were labeled or not.  I can't find any information on this other than
vague references.  The man page of mdadm isn't of any help here as well.

Does anyone know more about this or can give me ideas?



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