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I have a 2 disk mdadm RAID 0 volume, 4TB total, created in Ubuntu 14.04 server. The volume is less than half full and I'm looking to unraid the disks, keeping the data so that I can use one of the disks in another machine.

Ideally I'd like to be able to do this without buying another drive for data copy.

Thanks!

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If the raid array is partitioned, then you can use gparted to shrink and/or move the filesystems within it until the second half of the disk is unused. If it is not partitioned, then you need to shrink the filesystem with resize2fs ( assuming it is ext[234] ). Give yourself a little extra room just to be sure. Then you can convert the raid0 into a degraded raid4, which supports reshaping ( raid0 does not, but a degraded raid4 is essentially the same thing as a raid0 ). Then you can reshape the array down to only one disk, and remove the second disk.

resize2fs /dev/md2 2800M  # 2 3gb drives, left an extra 200mb of margin
mdadm -G /dev/md2 -l raid4
mdadm -G /dev/md2 -Z 2900M
e2fsck -f /dev/md2 # make sure you didn't break things
mdadm -G /dev/md2 -n 2 --backup=backup
mdadm -D /dev/md2 # watch until it is done reshaping
mdadm -G /dev/md2 -l raid0 # drop the second disk and go back to ( single disk ) raid0
mdadm -G /dev/md2 -Z max
resize2fs /dev/md2  # maximize the fs to use the full disk

In the process of testing this, I found that resize2fs would not let me shrink the fs to half of its original size even though it was only 33% used. I have submitted a patch to fix this minimum size calculation bug.

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  • I'm trying to do this with a /dev/md0 device that contains a /dev/md0p1 ext4 partition and a /dev/md0p2 linux-swap partition. When I resize the array (-Z option), the partitions disappear. They come back with -Z max. Any way to get rid of a disk with a setup like mine? – Jakob Kruse Sep 5 '16 at 18:37
  • @JakobKruse, you follow the instructions in the answer? – psusi Oct 7 '16 at 17:58
  • I did, and it didn't work. Breaks on step 3. I ended up imaging the ext4 partition to an external drive, zapping everything and then restoring the image. – Jakob Kruse Oct 12 '16 at 6:29
  • @JakobKruse, oh I see, you partitioned your raid array. I don't do that. In that case you would need to shrink/move the partitions in the array before you shrink the array itself. – psusi Oct 20 '16 at 23:21
  • Nice, wasn't aware about the raid4 reshaping feature, that's awesome. Well explained too. – John Jan 8 at 2:31

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