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I'm pretty new to RAID, so please forgive my silly question.
I have an Ubuntu Server 12.04 x64 installed on a pc having 4 hdds.
During RAID setup I selected RAID10 and (my ignorance) choose 2 active hdds and 2 spare hdds. I'm pretty sure it's not the optimal configuration and probably I should have selected 4 active hdds.

Running cat /proc/mdstat I get

md1 : active raid10 sdb2[1] sdd2[3](S) sdc2[2](S) sda2[0]
      948736 blocks super 1.2 2 near-copies [2/2] [UU]

md0 : active raid10 sdb1[1] sdd1[3](S) sdc1[2](S) sda1[0]
      311619072 blocks super 1.2 2 near-copies [2/2] [UU]

unused devices: <none>

Is there a way I can convert spare disks to active disks?
I tried using mdadm --grow /dev/md1 --raid-devices=4, but got this answer

mdadm: RAID10 can only be changed to RAID0
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1 Answer 1

up vote 1 down vote accepted

You cannot grow a RAID1+0 (it's not supported by mdadm).

From mdadm manpage:

Grow (or shrink) an array, or otherwise reshape it in some way. Currently supported growth options including changing the active size of component devices and changing the number of active devices in Linear and RAID levels 0/1/4/5/6, changing the RAID level between 0, 1, 5, and 6, and between 0 and 10, changing the chunk size and layout for RAID 0,4,5,6, as well as adding or removing a write-intent bitmap.

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So I cannot do anything else than keep current situation or restart from beginning, correct? :( –  Marco Sep 4 '13 at 9:00
    
on very recent kernels (>3.0) you may be able to convert raid10 to raid0, but it's a risky business. Maybe someone can chime in on this. –  Cubiq Sep 4 '13 at 9:03
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