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I have a disk of 500GB (465GB formatted) and one of 80GB (72GB formatted), and I wanted to create a linear array i.e. append the space of both disks with software-managed RAID tool mdadm. The bigger disk has more than 90% occupied and I would want to append the capacity of the smaller disk to it. Can I do it without wiping/backing up my bigger disk? It's not possible for me to back it up. I tried the following command

mdadm --create --verbose /dev/md0 --level=linear --raid-devices=2 /dev/sdb /dev/sdc

within a virtual machine. It creates /dev/md0 as a new block device, but it appears to need format, which, obviously is not what I want to do. Any ideas?

Also, I tried this over xubuntu16, but I plan to deploy it over debian 8. I have used mdadm on both of them and it does not appear to be any relevant difference.

EDIT: forgot to mention that I'm using NTFS over the main disk.

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LVM might be a better choice for linear mappings. There's a program that claims to be able to convert filesystems to LVM in place.

Some time ago I wrote a howto do this thing manually, it's in German:

If you don't care about anything, you could do it without metadata by using dmsetup directly to create a linear mapping. However this will confuse your system a lot.

Also, if I still had a 80GB disk, I would probably keep it separate. Merging disks together has a lot of downsides, basically one disk dies, and makes the data on the other disk useless as well. Because most filesystems don't like it at all when a segment suddenly goes missing.

You can have two separate filesystems and mount them in such a way that it seems like one filesystem. If you have a subdir in your 500GB disk that is just the right size (about 80GB) you can just move those files over and mount it in the appropriate place.

Or just treat them as separate disks outright, which they are after all. Keep things simple.

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I don't think you can pool disks while keeping the data.

In your case, with different drives, it would probably be best to create a disk pool. ZFS (or btrfs) will be your friend there.

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    Would you mind detailing a little more the process I should follow? – mrbolichi Oct 24 '16 at 12:56
  • Certainly, I'd be glad to, but better let someone else answer as well as I am not 100% sure that it cannot be done. I wanted to leave this as a comment, but I lack the reputation to do so. – fragamemnon Oct 24 '16 at 13:01

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