Growing a RAID6 array on 11.10 *** SOLVED ***

[Dave Lawrence]

On 09/03/12 00:09, Steven Davies-Morris wrote:
> On 03/07/2012 02:49 PM, Steven Davies-Morris wrote:
>> I have two RAID arrays on a system. The first is RAID1 and
>> supports /boot, swap / and /home. The second is RAID6 and
>> supports everything else. It currently has 4 3TB drives. I have 3
>> more sitting in the tower and want to add them to the RAID6
>> array.
>> 
>> I was using Disk Utility to try to add these three drives but it 
>> doesn't seem to want to do it with the array up and running. When
>> I take the array down I'm denied access to the array controls in
>> "Edit Components" so I can't tell it to add drives.
>> 
>> I remove the partitions from the 3 drives (initialized as GPT).
>> Then I go into "Edit Components". I see the three drives. I am
>> told they each have no partitions and each have 3TB available for
>> use.
>> 
>> I go to add one drive. I click the box to add it and the click 
>> "Expand". I authenticate then get this message:
>> 
>> Error expanding RAID Array. An error occurred while performing
>> on operation on "hitchcock RAID6" (6.0 TB RAID-6 Array): The
>> operation failed
>> 
>> Detail information says:
>> 
>> Error expanding array: helper script exited with exit code 1:
>> mdadm: added /dev/sdc1 mdadm: Need to backup 6144K of critical
>> section..
>> 
>> I now see the new drive in the RAID6 array as a spare.
>> 
>> What am I missing here? If I can't figure this out I guess it
>> will be a reboot into a run-level 1 root shell to douse mdadm.
>> Any pointers from those of you who are quite familiar with using
>> Disk Utility and RAID arrays?
> 
> I finally figured out what was going on. Digging further into the
> error message output I kept looking at the issue of bitmap removal.
> Why was this necessary? And removal from where? I had an epiphany
> 30 mins ago reading this mdadm man page:
> 
> <http://linux.die.net/man/8/mdadm>
> 
> Which then led me to search online and read this page which has
> some good information about speeding up working with Linux software
> raid:
> 
> <http://www.coderetard.com/2011/02/01/howto-speed-up-linux-software-raid-building-and-re-syncing/>
>
> 
> 
> What I wound up doing was removing the bitmap from the array with
> this shell command:
> 
> sudo mdadm --grow --bitmap=none /dev/md127
> 
> I was then able to expand the array using Disk Utility, though I
> could have done it from a terminal shell with this command:
> 
> sudo mdadm --grow /dev/md127 --raid-devices=7
> 
> Once I'm done with the reshaping I'll put the bitmap back after
> figuring out what it should optimally be. The RAID6 array is
> reshaping right now with the three new 3TB drives added. In a few
> hours I will seal it all back up. In the meantime I'm off to Fry's
> Electronics to return the extra pieces of hardware that I don't
> need.
> 
> The final configuration of the media center server will be a 8gb
> DDR3 machine with a AMD Phenom II X4 quad-core processor. It will
> have a RAID1 mirror of 2*2TB drives for boot, swap, / and home. And
> 7*3TB drives for the RAID6 array. 4 will be on the mobo along with
> the boot mirror. The other 3 drives will be on a 4port SATA card. 6
> drives are mounted in the 3.5" drive bays and the other 3 plus the
> PATA DVD burner are mounted in the 5.25" drive bays. 4 big fans,
> plus 1 giant fan and a 1200watt power-supply. Hopefully I'm now
> done tinkering with it and can get back to software and music.

Thanks for sharing the solution.  When I read your first post I had
thought the answer would be much more complex than that for RAID 6.
Just one tip to add: before reshaping RAID arrays, repartitioning etc
etc it's a good idea to have a practice run on a Virtual machine.