The mdadm tool was used to create a software RAID mirror using two device-mapper-multipath devices:
# /sbin/mdadm /dev/md0 --create --verbose --level=1 --raid-devices=2 /dev/mapper/ocrp1 /dev/mapper/ocrmirrorp1
The setup was then confirmed:
# /sbin/mdadm --detail /dev/md0 ... Number Major Minor RaidDevice State 0 253 2 0 active sync /dev/dm-2 1 253 3 1 active sync /dev/dm-3
Since the actual multipath device names (/dev/dm-N) are shown, the mappings of the more-friendly names (/dev/mapped/ocrp1) are also verified:
# /bin/ls -l /dev/mpath/ lrwxrwxrwx 1 root root 7 Apr 23 11:15 ocrp1 -> ../dm-2 lrwxrwxrwx 1 root root 7 Apr 23 11:15 ocrmirrorp1 -> ../dm-3
# /bin/ls -l /dev/mapper/ brw-rw---- 1 root disk 253, 2 Apr 23 11:15 ocrp1 brw-rw---- 1 root disk 253, 3 Apr 23 11:15 ocrmirrorp1
Since these convenient names map to the same multipath devices, the setup has been proven correct. After a reboot, mdadm shows the following:
# /sbin/mdadm --detail /dev/md0 ... Number Major Minor RaidDevice State 0 8 97 0 active sync /dev/sdg1 1 8 113 1 active sync /dev/sdh1
The RAID is active but is not using the multipath devices as expected.
This is actually a timing problem. During the system boot, the /etc/rcN.d scripts are starting mdadm before the multipath devices have been detected and ready. This is essentially a race condition because a larger number of multipath devices take longer to recognize and mdadm may be run before the multipath processing is complete.
The solution is to add the requisite file system handlers and devices into the /initrd file so that these will be available to the kernel at boot-time. This will allow the kernel to start processing the multipath devices earlier.
This clever approach allows one kernel image to be provided with a device driver set tailored for each system, without wasting system memory to hold device driver and file system handler code which is never used.
To build a custom /initrd file including the multipath support, use the below technique:
1. Create a new initrd file including the multipath, device-mapper-multipath, and HBA driver:
# /sbin/mkinitrd -v /root/initrd-mp.img 2.6.18-prep --with=multipath --with=dm-multipath --with=lpfc --omit-raid-modules
To do the same on CentOS/RHEL 6 and 7, please have a look at below post.
2. Verify the file /etc/mdadm.conf is configured for the RAID device:
# /bin/cat /etc/mdadm.conf DEVICE /dev/mapper/* ARRAY /dev/md0 uuid=ccfe8a98:ea584ff2:2fad9d51:305ea2da devices=/dev/mapper/ocrp1,/dev/mapper/ocrmirrorp1 level=raid1
3. Copy the new ramdisk image into the boot location expected by the bootloader:
# /bin/cp /root/initrd-mp.img /boot/
4. Add a new entry to the bootloader configuration file /boot/grub/grub.conf to use the new ramdisk image:
title MDADM-MP root (hd0,0) kernel /vmlinuz-2.6.18-8.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet initrd /initrd-mp.img
When using CentOS/RHEL 7 and grub2, please see below post for exact steps.
5. Reboot your system and select the MDADM-MP entry from the GRUB menu.