To prevent this issue, three steps are required:

• change the order kernel modules get loaded by initrd
• disable the mdadm auto-assembly of disks when initrd is loaded
• generate a new initrd image

The initrd configuration files can be found at /etc/initramfs-tools.

First define the order of module loading during boot (ensure that multipath is loaded before mdadm). Edit the file /etc/initramfs-tools/modules to match:

# List of modules that you want to include in your initramfs.
# They will be loaded at boot time in the order below.
#
# Syntax: module_name [args ...]
#
# You must run update-initramfs(8) to effect this change.
#
# Examples:
#
# raid1
# sd_mod
libfc
megaraid_sas
scsi_dh_alua
scsi_transport_fc
dm_multipath
dm_service_time
multipath
md_mod

This starts by loading the “fiber channel libs” (libfc) first, followed by the MegaRAID modules, SCSI over fiber channel, multipath and mdadm.

Now copy the mdadm initramfs hook to /etc to override the default one:

cp /usr/share/initramfs-tools/hooks/mdadm /etc/initramfs-tools/hooks/

Edit the hook file in /etc/initramfs-tools/hooks/mdadm and add an ‘exit 0‘ (see line 10):

#!/bin/sh
#
# Copyright © 2006-2008 Martin F. Krafft <madduck@debian.org>,
#
2012 Michael Tokarev <mjt@tls.msk.ru>
# based on the scripts in the initramfs-tools package.
# released under the terms of the Artistic Licence.
#
set -eu
exit 0
PREREQ="udev"
prereqs()
{
echo "$PREREQ"
}

Disable automatic startup of the mdadm service:

rm /lib/systemd/system/mdadm.service
systemctl daemon-reload
systemctl disable mdadm.service

Finally, update the initrd images for all kernel versions installed:

update-initramfs -u -k all

Reboot the server to see if modules are now loaded in the correct order, and multipath is loaded and started before mdadm.