Hi Neil,
Thanks for the Info, I'll try a new series of VM tomorrow.
I do have a question though. I thought that RAID5 required 3 HDD not 2.
Hence I am a bit puzzled by your last comment....
"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. " Or do I grow to a 3HDD RAID5 config with a 'missing' HDD.
I understand the 2HDD to 5HDD growth, but not how to make the other one.
Since I cant test it right know, I'll both tomorrow.
Dom
On 01/10/2011 00:02, NeilBrown wrote:
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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