My array is built of 3 hard drive of 2TB, I have 2 more free space and I would love to start to grow my array with 4TB hard drives, if I start to put 2 4TB hard drives and eventualy replace the 2TB hard drives and grow the arry too!

Is it possible?

I'm using mdadm on a Ubuntu server 14.04!


As far as I know, and as supported by this article, you can setup a RAID array of different size disks, but the array will end up sized as Number of Disks * Smallest Disk.

Can I create an md(4) RAID array out of block devices of unequal size?
Yes, but the RAID array will have the size of the smallest block device (plus some overheads for its own housekeeping). If device sizes aren't within 1% of each other, you get a warning.

This concept also applies to hardware RAID controllers like HP's SmartArray line.

The article referened above shows you how to test the theory by creating dummy devices and then layering md devices on top.

  • This is, in fact, one of the mentioned methods for expanding an array, see the Expanding a partiton section of the mdadm wiki. Just keep in mind for most of these steps a full backup of the RAID is best, as the RAID is vulnerable during resyncing. – Reaces Mar 26 '15 at 11:52

No. In a raid, every member drive must be the same size, it is unavoidable. Otherwise, where could be mapped the upper 2TB of your new drives?

But, what you can do:

  1. Partition your new drives to have two-two 2TB partitions,
  2. Grow your old array to include the lower 2-2 TB partitions from your disks (mdadm --grow /dev/md0 --raid-devices=5)
  3. And finally, you will have 2-2 free TBs on your new disks.

It will be sometimes a little bit uncommon configuration, but it will work.

  • Oh, I really love your idea! I future, if we finish to have 6TB drives, I'll do the same, make 3 partitions of 2TB and get 3 more drives in the array! ;) Thanks! – TheBuzzer67 Mar 27 '15 at 5:06
  • @TheBuzzer67 This wasn't what I wrote. Maybe try it again. – peterh - Reinstate Monica Mar 27 '15 at 10:07

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