software raid auto assembly problem on Xen host

Nicholas Leippe nick at
Wed Mar 3 12:54:56 MST 2010

On Mon, Mar 1, 2010 at 5:39 PM, Nicholas Leippe <leippe at> wrote:
> On Mon, Mar 1, 2010 at 5:31 PM, Michael Torrie <torriem at> wrote:
>> 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?
> You can pass raid=noautodetect to the kernel to prevent it from
> enumerating any md devices.

The option you're looking for is --homehost= during array creation.
It affects the metadata, and depending on the superblock version you
chose the UUID.
I don't know if you can change it after-the-fact.
I do know that only 0.90 superblocks is supported for some operations
(booting off of raid1 I believe is one of them), despite the 1.0
version being several years old and improved in a few ways. Go figure.

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