I got "mdadm: can only add devices to linear arrays" when I did not expect it. Can anyone explain what's going on, and what the right way to handle the situation is?
In brief, I had a raid1 set for 2 devices, but with only 1. I tried adding a 2nd.
root@tempserver:/var/log# mdadm -D /dev/md/media4
/dev/md/media4:
Version : 1.2
Creation Time : Thu Feb 18 14:09:00 2016
Raid Level : raid1
Array Size : 1953376064 (1862.88 GiB 2000.26 GB)
Used Dev Size : 1953376064 (1862.88 GiB 2000.26 GB)
Raid Devices : 2
Total Devices : 1
Persistence : Superblock is persistent
Update Time : Fri Feb 17 17:59:40 2017
State : clean, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0
Name : tempserver:media4 (local to host tempserver)
UUID : 69440821:e9aa9259:ab2b06ce:d09bedc7
Events : 555297
Number Major Minor RaidDevice State
0 0 0 0 removed
2 8 34 1 active sync /dev/sdc2
root@tempserver:/var/log# mdadm --grow /dev/md/media4 --add /dev/sda2
mdadm: can only add devices to linear arrays
# I figured since it was raid1 with a missing device it would use the one I added
root@tempserver:/var/log# mdadm --grow /dev/md/media4 --add /dev/sda2 -n 2
mdadm: /dev/md/media4: no change requested
root@tempserver:/var/log# date; mdadm --grow /dev/md/media4 -n 1
# so I reduced the size of the array. I was a little worried this would kick sdc2 out of the array.
# It did not. I then was able to add
# the new partition in what I think is the usual way.
root@tempserver:/var/log# date; mdadm --grow /dev/md/media4 --force -n 1
Fri Feb 17 18:18:28 PST 2017
raid_disks for /dev/md/media4 set to 1
root@tempserver:/var/log# mdadm -D /dev/md/media4
/dev/md/media4:
Version : 1.2
Creation Time : Thu Feb 18 14:09:00 2016
Raid Level : raid1
Array Size : 1953376064 (1862.88 GiB 2000.26 GB)
Used Dev Size : 1953376064 (1862.88 GiB 2000.26 GB)
Raid Devices : 1
Total Devices : 1
Persistence : Superblock is persistent
Update Time : Fri Feb 17 18:18:41 2017
State : clean
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0
Name : tempserver:media4 (local to host tempserver)
UUID : 69440821:e9aa9259:ab2b06ce:d09bedc7
Events : 557442
Number Major Minor RaidDevice State
2 8 34 0 active sync /dev/sdc2
root@tempserver:/var/log# date; mdadm --grow /dev/md/media4 --add /dev/sda2 -n 2
Fri Feb 17 18:19:24 PST 2017
mdadm: added /dev/sda2
raid_disks for /dev/md/media4 set to 2
# and mdadm -D shows the array rebuilding with sda2 being rebuilt.
Thanks.
Ross Boylan--
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