Hi! I’m learning how to use btrfs and I need some advice.

One one of my desktop, I made the mistake of creating 2 partitions, one for /(root) and one for home. Both are btrfs. I didn’t know that I could use subvolumes so that they could share the same physical space.

My question: How can I merge the root and home btrfs partitions into only 1 partition that would use btrfs subvolumes?

I’m looking for something like that:

  • Partition1 (btrfs)
    • subvolume 1: @root (mounted to /)
    • subvolume 2: @home (mounted to /home)
  • Partition 2, 3, 4…

My current setup:

  • 1 physical hard drive (1 TB), shown as sda below
  • The partitions I want to merge are sda7 and sda8
  • That computer is an iMac also running MacOS so it has a few other partitions that I should not touch
$ lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
sda      8:0    0 931,5G  0 disk 
├─sda1   8:1    0   200M  0 part /boot/efi
├─sda2   8:2    0 371,1G  0 part 
├─sda3   8:3    0 619,9M  0 part 
├─sda4   8:4    0   600M  0 part 
├─sda5   8:5    0  1023M  0 part 
├─sda7   8:7    0 422,9G  0 part /home
└─sda8   8:8    0 135,1G  0 part /
sdb      8:16   1     0B  0 disk 
sr0     11:0    1  1024M  0 rom  

$ blkid
/dev/sda4: UUID="d970eea2-142b-3f1c-9650-5e496d1e9b4b" BLOCK_SIZE="4096" LABEL="Linux HFS+ ESP" TYPE="hfsplus" PARTLABEL="Linux HFS+ ESP" PARTUUID="eab00592-b96d-4ecb-b2e9-816c95eaf860"
/dev/sda2: UUID="6a26963c-eabb-3e42-9e8d-8677a8282b61" BLOCK_SIZE="4096" LABEL="DD Macintosh" TYPE="hfsplus" PARTLABEL="DD Mac" PARTUUID="8257316c-1fd7-4885-bf2b-7e99557acd85"
/dev/sda7: UUID="22f5e59e-8509-484a-92d5-e7dc03bb70cd" UUID_SUB="78a998f5-db55-4a31-9506-afe548ec8d5e" BLOCK_SIZE="4096" TYPE="btrfs" PARTLABEL="Mint Home" PARTUUID="20b6c31d-5e2d-416f-beb3-faa295af67df"
/dev/sda5: UUID="587b0093-4b64-468a-9a01-b933630d184b" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="a47a3fbf-534a-435d-8916-0f83edebf296"
/dev/sda3: UUID="d0c171e8-572d-39f9-8bdc-38f33744a19a" BLOCK_SIZE="4096" LABEL="Recovery HD" TYPE="hfsplus" PARTLABEL="Recovery HD" PARTUUID="c9d0673b-bb2e-4322-9bf9-c661f7de6856"
/dev/sda1: LABEL_FATBOOT="EFI" LABEL="EFI" UUID="67E3-17ED" BLOCK_SIZE="512" TYPE="vfat" PARTLABEL="EFI System Partition" PARTUUID="d30954cb-b9b6-40fa-9202-a18cf146f7df"
/dev/sda8: UUID="7027382a-4369-4276-b916-9997c1007e5b" UUID_SUB="3e516464-dee1-4bda-9621-29591d54dc2d" BLOCK_SIZE="4096" TYPE="btrfs" PARTLABEL="Mint root" PARTUUID="f2fdb8cc-54fd-46c8-bf1f-85954c1dc363"
  • @seaQueue
    1•11 hours ago

    Assuming you have no subvolumes on either partition I would boot a live USB system and mount them then take snapshots of each. Send/receive the home snapshot to the root filesystem, remove the home filesystem and then extend the root partition to fill all space.

    Once that’s done mount the root snapshot (which is a subvolume in its own right, all snapshots are) and fix up your fstab to point at the new filesystem and subvolume names. Fix your boot cmdline as well using rootflags=subvol=SUBVNAME and if needed update GRUB.

    Once you can successfully boot the system using the new subvolumes then (and only then) mount the root of the remaining btrfs partition and remove all of the old directories and files outside of your subvolumes.

    A little tedious sure but easy enough with a bit of time.