HowTo Single Disk to RAID
We found ourselves in the need of switching from a single disk root to a root backed by a RAID, here's how we did.
Context
We sat up a new/old server, BitArno, it has a nice /home of ~6TB backed by a RAID and a separate dick for the / partition. As the drive for the / was tested before getting to production resulting in no errors we tought we didn't need a RAID for it. We installed debian stable 8.1, sat up every service, went to bet to wake up with a hung server refusing to boot.
An on site check was required, we got there and found out that the / drive was failing to reallocate sectors, we fsck'ed it and it booted. "Great!" that same evening it was failing again and so we decided to switch from a single disk root to a RAID1 backed root expecially because we do use commodity hardware taken from old computers.
The general idea
We wanted a way to easily switch from a drive to the RAID without having to back everything up and copy it over again with fewer downtime possible and so we decided to:
- Poweroff the server and attach the new disk of an equal size
- Power on the server
- Create a RAID1 array composed by 2 drives but with a drive marked as missing
- Set LVM up on top of it (as the old drive had LVM (it's a 120GB drive just for the `/` parition, we wanted to have some space available if we'll ever find in the need)
- Rsync the files on the new disk (member of the array)
- Install the bootloader on the new drive adding the necessary modules for RAID and LVM
- Reboot the server from the RAID-memeber disk
- Check if everything is OK
- Add the old disk to the RAID and let mdadm resync the contents.
Step by step guide
Here I'll try to cover step by step the command we used, it has been a while since we did it so I'm not 100% sure I'll remember everything First a layout of the partition befor the RAID
sda 8:0 0 114.5G 0 disk ├─sda1 8:1 0 243M 0 part /boot ├─sda2 8:2 0 1K 0 part └─sda5 8:5 0 114.3G 0 part ├─ba--system-root 253:0 0 30G 0 lvm / └─ba--system-swap 253:3 0 4G 0 lvm [SWAP]
And after the switch
sda 8:0 0 114.5G 0 disk
├─sda1 8:1 0 243M 0 part
│ └─md1 9:1 0 242.8M 0 raid1 /boot
├─sda2 8:2 0 1K 0 part
└─sda5 8:5 0 114.3G 0 part
└─md2 9:2 0 114.2G 0 raid1
├─ba--system-root 253:0 0 30G 0 lvm /
└─ba--system-swap 253:3 0 4G 0 lvm [SWAP]
sde 8:64 0 114.5G 0 disk
├─sde1 8:65 0 243M 0 part
│ └─md1 9:1 0 242.8M 0 raid1 /boot
├─sde2 8:66 0 1K 0 part
└─sde5 8:69 0 114.3G 0 part
└─md2 9:2 0 114.2G 0 raid1
├─ba--system-root 253:0 0 30G 0 lvm /
└─ba--system-swap 253:3 0 4G 0 lvm [SWAP]
In this way we have two arrays, one for the boot and one for the LVM which contains the root and the swap
First of all we need to wipe out every possible header from the new disk as it will become a RAID member
dd if=/dev/zero of=/dev/sde bs=100M count=1
Then we created the partitions with sfdisk about the same size as the first disk. It's not needed for them to be the same as it wasn't possible to copy the partitions with dd
We need:
- one parition for the boot marked as Linux raid autodetect
- the other for LVM marked as Linux raid autodetect that takes the free space left
We then proceeded to create the 2 RAIDS, /dev/sde is the new disk
mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/sde1 missing mdadm --create /dev/md2 --level=1 --raid-devices=2 /dev/sde5 missing
Issues
We (of course) had some issues in the process:
- Thougth the drive was of the same model the capacity was a little different and so we couldnt copy the partition scheme as we tougth we could do
- It's very important to generate the grub image inserting the modules and the boot parameter needed for lvm and RAID
- It's impossible to backup and restore the lvm too as for a while we found ourselves with the 2 LVMs disk together and names were conflicting.