LVM and raid

[Matt Graham]

On 2015-07-21 13:28, Stephen Partington wrote:
> Ubuntu 15.04 installed and am looking to find out if there is a way
> to migrate to raid on a running system.
> 
> In theory i should be able to do this by creating a degraded portion
> of the raid volume on the empty disk, extend/move my LVM to that disk
> then rebuild the original disk as part of that raid volume.

This is a softRAID-1, right?  That makes the most sense for what you've 
said.

> I was wondering if anyone had some documentation of information i
> could read about this scenario.

I did something like this when I moved my stuff from just 1 disk on 
regular partitions to softRAID-1.  First, back up all your junk, because 
there are multiple points in this where you could scribble all over your 
disks.  Second, make sure you have a LiveCD or LiveUSB disk to boot from 
if the bootloader goes wonky.  
https://wiki.gentoo.org/wiki/Complete_Handbook/Software_RAID should be 
pretty applicable to working with softRAID and applicable to distros 
that are not Gentoo.

You'd first take the new disk and partition it.  I would think you'd 
need at least 2 partitions since having /boot on LVM is not going to 
work, and possibly 3 because having / on LVM has more fiddly bits than 
having it on a regular partition.

So, fdisk /dev/sdb , set it up with 2 or 3 partitions (whichever), and 
then set up the RAID.  This'll assume that your largest partition (the 
one that'll be your PV) is /dev/sdb2 .  You'd then do

mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb2 missing

This'll set up an array in degraded mode on /dev/md0.  You'd then 
pvcreate /dev/md0 , then vgextend $VG_NAME /dev/md0 , then pvmove 
/dev/$OLD_PV_LOCATION .  Then vgreduce $VG_NAME /dev/$OLD_PV_LOCATION  
and pvremove /dev/$OLD_PV_LOCATION to remove the old PV from lvm's 
config.  Then you'd add the old PV back to the md0 with mdadm /dev/md0 
--add /dev/$OLD_PV_LOCATION .  The disk sync will probably take a while.

You'll have to set things up so that the bootloader can figure 
everything out.  Are you going to put / on LVM?  That requires that the 
initrd have all the LVM tools on it.  I don't know if Ubuntu can handle 
that automatically or not--I would guess so, but ICBW.

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