On Fri, 30 Sep 2011 20:31:37 +0200 Dominique <dcouot@xxxxxxxxxxx> wrote:
> Hi,
>
> Using Ubuntu 11.10 server , I am testing RAID level changes through
> MDADM. The objective is to migrate RAID 1 (1+ HDD) environment to RAID 5
> (3+ HDD) without data loss.
> In order to make as simple as possible, I started in a VM environment
> (Virtual Box).
Very sensible!!
>
> Initial Setup:
> U11.10 + 2 HDD (20GB) in Raid 1 -> no problem
> The setup is made with 3 RAID 1 partition on each disk (swap (2GB), boot
> (500MB), and root (17,5GB)). I understand that this will allow to
> eventually grow to a RAID 5 configuration (in Ubuntu) and maintain boot
> on a RAID construct (swap and boot would remain on RAID 1, while root
> would migrate to RAID 5).
>
> Increment number of disks:
> add 3 HDD to the setup -> no problem
> increase the RAID 1 from 2 HDD to 5 HDD -> no problem, all disks added
> and synchronized
This is the bit you don't want. Skip that step and it should work.
>
> root@ubuntu:~# cat /proc/mdstat
> Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5]
> [raid4] [raid10]
> md2 : active raid1 sda3[0] sde3[4] sdb3[1] sdc3[2] sdd3[3]
> 18528184 blocks super 1.2 [5/5] [UUUUU]
>
> md1 : active raid1 sda2[0] sde2[4] sdb2[1] sdd2[3] sdc2[2]
> 488436 blocks super 1.2 [5/5] [UUUUU]
>
> md0 : active raid1 sdb1[1] sde1[4] sda1[0] sdc1[2] sdd1[3]
> 1950708 blocks super 1.2 [5/5] [UUUUU]
>
>
> Change Level:
> That's where the problem occurs:
> I initially tried 3 different approaches for md2 (the root partition)
>
> 1. Normal boot
>
> mdadm /dev/md2 --grow --level=5
>
> Not working: 'Could not set level to raid 5'. I suppose this is
> because the partition is in use. Makes sense.
Nope. This is because md won't change a 5-device RAID1 to RAID5. It will
only change a 2-device RAID1 to RAID5. This is trivial to do because a
2-device RAID1 and a 2-device RAID5 have data in exactly the same places.
Then you can change your 2-device RAID5 to a 5-device RAID5 - which takes a
while but this can all be done while the partition is in use.
i.e. if you start with a RAID1 with 2 active devices and 3 spares and issue
the command
mdadm /dev/md2 --grow --level=5 --raid-disks=5
it will convert to RAID5 and then start reshaping out to include all 5 disks.
NeilBrown
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