Randy's Blog

Historically XenServer does not support software RAID out-the-box, and this is unchanged in the latest 6.2 release. We can convert it to RAID after installation.

First we set up the 2nd disk by copying the partition tables whilst enabling a degraded RAID on it, then copy data over with custom initrd.

sgdisk -R/dev/sdb /dev/sda  # Replicate partion table from /dev/sda to /dev/sdb with unique identifier
sgdisk --typecode=1:fd00 /dev/sdb
sgdisk --typecode=2:fd00 /dev/sdb
sgdisk --typecode=3:fd00 /dev/sdb
# Sleep 5 seconds here if you script this...
yes|mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 missing # Create md0 (root)
yes|mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/sdb2 missing # Create md0 (swap)
yes|mdadm --create /dev/md2 --level=1 --raid-devices=2 /dev/sdb3 missing # Create md0 (storage)
# Sleep 5 seconds here if you script this...
mkfs.ext3 /dev/md0 # Create root FS
mount /dev/md0 /mnt # Mount root FS
cp -xR --preserve=all / /mnt # Replicate root files
sed -i 's/LABEL=[a-zA-Z\-]*/\/dev\/md0/' /mnt/etc/fstab # Update fstab for new RAID device
mkdir /mnt/root/initrd-raid && cd /mnt/root/initrd-raid
mkinitrd -v --fstab=/mnt/etc/fstab /mnt/root/initrd-raid/initrd-`uname -r`-raid.img `uname -r`
zcat initrd-`uname -r`-raid.img | cpio -i
sed -i 's/raidautorun \/dev\/md0/raidautorun \/dev\/md0\nraidautorun \/dev\/md1\nraidautorun \/dev\/md2/' init
rm -f initrd-`uname -r`-raid.img
find . -print | cpio -o -Hnewc | gzip -c > /mnt/boot/initrd-`uname -r`-raid.img
rm -f /mnt/boot/initrd-2.6-xen.img
ln -s initrd-`uname -r`-raid.img /mnt/boot/initrd-2.6-xen.img
sed -i 's/LABEL=[a-zA-Z\-]*/\/dev\/md0/' /mnt/boot/extlinux.conf
cat /mnt/usr/share/syslinux/gptmbr.bin > /dev/sdb
cd /mnt && extlinux  --raid -i boot/
sgdisk /dev/sda --attributes=1:set:2
sgdisk /dev/sdb --attributes=1:set:2
cd && umount /dev/md0
sync
reboot

Now, make sure BIOS forces booting from 2nd disk. This is VERY important! After reboot, finish off:

sgdisk -R/dev/sda /dev/sdb  # Replicate partion table from /dev/sdb to /dev/sda with unique identifier
mdadm -a /dev/md0 /dev/sda1
mdadm -a /dev/md1 /dev/sda2
mdadm -a /dev/md2 /dev/sda3  # If this command gives error, you need to forget/destroy an active SR first
mdadm --detail --scan > /etc/mdadm.conf
xe sr-create content-type=user device-config:device=/dev/md2 host-uuid=[host_uuid] name-label="Local Storage" shared=false type=lvm

Before going into production, try booting from each disk (with the other removed from the boot priorities), then restore the boot priority to normal.

Be careful with Xenserver patches, especially any patch which requires a reboot – if the initrd image is rewritten (for example, kernel updated), you need to carefully rebuild the initrd for RAID support again which is NOT covered in this article.

Sometimes a hardware RAID controller or fakeraid (BIOS) can leave metadata that makes it impossible to install Windows or Linux, or it installs correctly, but causes a kernel panic or a 0xb7 blue screen error on the first boot. The only method I could find to delete the metadata *quickly* is to zero out the last 512KB of data on the disk using the following command:

dd if=/dev/zero of=$YOUR_DEV bs=512 seek=$(( $(blockdev --getsz $YOUR_DEV) - 1024 )) count=1024

Replace $YOUR_DEV with the physical device, such as /dev/sda

You could just zero the whole disk, but that could take hours. This command executes in less than a second.