Setup SAN Boot for RHEL 6.x using native multipath on EMC storage

Requirements:
1) RHEL 6.x (most apply to RHEL 5.x too, RHEL 5.x use mkinitrd instead of Dracut and the /etc/multipath.conf is slightly different refer to Red Hat KB in reference section)
2) EMC storage was setup with Active/Active (ALUA)
3) Boot LUN was presented with single path for initial install


Procedures:

1. Server boots up after initial install
2. Login to server as root to enable multipath
[root@server1]#mpathconf --enable –-with_multipathd y
3. Edit /etc/multipath.conf and make sure it only contains following valid parameters

blacklist {
}


defaults {
 user_friendly_names yes
}
devices {
  device {
    vendor "DGC"
    product ".*"
    product_blacklist "LUNZ"
    hardware_handler "1 alua"   
    path_checker directio    
    prio alua                
  }
}

4.Find out the logical path the root disk is mapped
[root@server1]#multipath –v3
It should be /dev/mapper/mpatha

5. Create initramfs with multipath module
[root@server1]#dracut --force -–add multipath

6. Make sure multipath.conf is included in initrd image

[root@server1]#lsinitrd /boot/initramfs-*.x86_64.img | grep multipath.conf
-rw-r--r--   1 root     root         2525 Feb 27 13:31 etc/multipath.conf

7. Modify the /boot/grub/device.map and change

 (hd0) /dev/sda    to
 (hd0) /dev/mapper/mpatha

This is assuming the boot disk is on /dev/mapper/mpatha as verified in step 2 above.

8. Reboot the server.

9. Verify multipath, check hwhandler='1 alua' and member disk sda for mpatha

[root@server1]#multipath –ll 
mpatha (3600601609973310067eb1e1ed69ae311) dm-0 DGC,VRAID
size=150G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
`-+- policy='round-robin 0' prio=10 status=enabled
  |- 1:0:0:0 sda 8:0   active ready running
  

10. Ask storage administrator to enable other paths for boot LUN.
11. Reboot server again after multipath is aenabled in storage too
12. Login server to verify all paths, check hwhandler='1 alua' prio>0
if hwhandler='1 emc' or prio=0 means PNR mode

[root@server1]#multipath -ll
mpatha (3600601609973310067eb1e1ed69ae311) dm-0 DGC,VRAID
size=150G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
|-+- policy='round-robin 0' prio=130 status=active
| |- 1:0:1:0 sdd 8:48  active ready running
| `- 2:0:1:0 sdj 8:144 active ready running
`-+- policy='round-robin 0' prio=10 status=enabled
  |- 1:0:0:0 sda 8:0   active ready running
  `- 2:0:0:0 sdg 8:96  active ready running
mpathb (360060160997331009fd6e124d69ae311) dm-1 DGC,VRAID
size=800G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
|-+- policy='round-robin 0' prio=130 status=active
| |- 1:0:0:1 sdb 8:16  active ready running
| `- 2:0:0:1 sdh 8:112 active ready running
`-+- policy='round-robin 0' prio=10 status=enabled
  |- 1:0:1:1 sde 8:64  active ready running
  `- 2:0:1:1 sdk 8:160 active ready running

13. Partition other LUNS using fdisk command as normal, but use logical path /dev/mapper/mpathb etc (partition will be created as /dev/mapper/mpathbp1 instead of /dev/mapper/mpathb1
NOTE: any change to /etc/multipath.conf requires re-create initramfs ( dracut --force -–add multipath) and a reboot, because the boot LUN is on SAN, if boot LUN is local disk, change to /etc/multipath.conf only requires multipathd restart