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I have recently moved from a 2TB HDD with Windows 7 Enterprise (R) and several ext4 and XFS data partitions to a 4TB HDD by dd 'ing Windows to the new HDD.

When I wanted to create an extended partition over the left 3.5 TB the partitioner told me that I cannpt do that because MSDOS partition tables do not support such big partitions, so I created an 2TB btrfs partition (with several subvolumes for my data) and a 1.5TB extended partition.

Now I have an 900GB btrfs partition and a 2TB btrfs partition, how can I "join" the disk space. I thought that this would be possible using btrfs device add, however this seems to use RAID0, so I would essentialy loose some disk space, because 2*900GB = 1.8TB, while I already have 2TB.

Is there a way to add the second disk, so that I get 2.9TB disk space?

My Disk Setup

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2 Answers 2

Reset to default
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Your BTRFS defaulted to the raid0 mode when adding a new device; you can change it to single mode by running btrfs balance start -dconvert=single,soft /mountpoint.

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    Worked like a charm! Can you provide a link to some documentation where one can read a bit more about that feature?
    – LittleByBlue
    Commented Nov 25, 2018 at 17:47
  • Sure, the best info on BTRFS can be found on the official wiki: btrfs.wiki.kernel.org/index.php/… btrfs.wiki.kernel.org/index.php/Balance_Filters
    – Atemu
    Commented Nov 26, 2018 at 10:31
  • Well the official wiki does not describe what single actually means.
    – LittleByBlue
    Commented Nov 29, 2018 at 8:38
  • "The option -m single means that no duplication is done (...)"
    – Atemu
    Commented Nov 29, 2018 at 10:08
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(Indeed a comment, but a bit long. Sorry)

Update in btrfs-5.8 (in y2020):

  • $ mkfs.btrfs -d <profile> now defaults to single for multiple devices. Until 5.7, it was raid0, as this question and the answer were done.

In btrfs-5.15 (in y2021)

  • $ mkfs.btrfs -m <profile> defaults to dup for any case. Until 5.14, when it's SSD and for multiple devices, the default was single.
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