BTRFS - Problem adding new sub-volumes

[Qvo]

Hello,

I’ve read through a bunch of threads here and on other forums + the BTRFS manual but I still need assistance.

I want to create new subvolumes and mount existing folders to them (such as /run, /tmp & my own /scripts), but I can’t quite grasp how to do that.

Here is the current subvolume setup from installing Manjaro:

ID 256 gen 1718 top level 5 path @
ID 257 gen 1768 top level 5 path @home
ID 258 gen 1408 top level 5 path @cache
ID 259 gen 1768 top level 5 path @log

Here is my fstab:

UUID=0094-6C92                            /boot/efi      vfat    umask=0077 0 2
UUID=a651d5d2-ac20-40ae-a8ba-1b25be4e117d /              btrfs   subvol=/@,defaults,space_cache=v2,compress=zstd:3,discard=async,ssd 0 0
UUID=a651d5d2-ac20-40ae-a8ba-1b25be4e117d /home          btrfs   subvol=/@home,defaults,space_cache=v2,compress=zstd:3,discard=async,ssd 0 0
UUID=a651d5d2-ac20-40ae-a8ba-1b25be4e117d /var/cache     btrfs   subvol=/@cache,defaults,space_cache=v2,compress=zstd:3,discard=async,ssd 0 0
UUID=a651d5d2-ac20-40ae-a8ba-1b25be4e117d /var/log       btrfs   subvol=/@log,defaults,space_cache=v2,compress=zstd:3,discard=async,ssd 0 0
UUID=9579c81f-26f1-4874-b611-675b878f2633 swap           swap    defaults,noatime 0 0
tmpfs                                     /tmp           tmpfs   defaults,noatime,mode=1777 0 0

If I go to / and create a new sub-volume via “btrfs subvolume create scripts”(for example), it creates a new sub-volume and also adds a directory with the same name (scripts). If I create a sub-volume with the name [at]scripts…it’ll also add a directory with the name [at]scripts. Yet, I can’t find a folder called [at]home, [at]cache, or [at]log and I’m not able to figure out why that is (I get that they’re mounted at real directories per fstab layout though). I think they’re sub-volumes under the [at] sub-volume, so how do I create a new sub-volume that falls under [at]?

Additionally, if I want to mount a directory to a subvolume, do I need to copy the contents over from the original folder to the sub-volume created directory (via cp --ref-link=always) and then rename it to the same?

[megavolt]

@Qvo

If I go to / and create a new sub-volume via “btrfs subvolume create scripts”(for example), it creates a new sub-volume and also adds a directory with the same name (scripts).

Mount the partition without targeting a subvolume:

sudo mount -m -U a651d5d2-ac20-40ae-a8ba-1b25be4e117d /mnt/fstree

And create then the subvolume on the fs-tree.

sudo btrfs subvolume create /mnt/fstree/@scripts

Just for understanding:

sudo btrfs subvolume show /mnt/fstree 

will show Name: <FS_TREE>, while

sudo btrfs subvolume show /

will show Name: @

@Qvo

Additionally, if I want to mount a directory to a subvolume, do I need to copy the contents over from the original folder to the sub-volume created directory (via cp --ref-link=always) and then rename it to the same?

What? You want to mount a dir to a subvol ? Shouldn’t it be the way around? What do you try to accomplish here?

[Aragorn]

@Qvo

I want to create new subvolumes and mount existing folders to them (such as /run, /tmp & my own /scripts)

/run and /tmp are both tmpfs, i.e. filesystems in virtual memory. They do not contain anything unless the system is running, and their contents do not exist on any physical drive.

@Qvo

If I create a sub-volume with the name [at]scripts…it’ll also add a directory with the name [at]scripts. Yet, I can’t find a folder called [at]home, [at]cache, or [at]log and I’m not able to figure out why that is (I get that they’re mounted at real directories per fstab layout though). I think they’re sub-volumes under the [at] sub-volume, so how do I create a new sub-volume that falls under [at]?

The @ signifies the root subvolume of the btrfs filesystem — note: this is not necessarily the same thing as the root directory — and need not be specified when creating a subvolume.

sudo btrfs subvolume create /path/to/name

@Qvo

Additionally, if I want to mount a directory to a subvolume, do I need to copy the contents over from the original folder to the sub-volume created directory (via cp --ref-link=always) and then rename it to the same?

Rename the original directory (“folder”), create the subvolume with the name it should have and where it needs to go, mount the subvolume to its own directory — created when the subvolume itself is created — and move over the contents of the original directory to the mountpoint.

Also make sure that the new mountpoint is listed in /etc/fstab.

[Qvo]

Thanks! I couldn’t figure out that I needed to mount the partition directly. Problem’s solved! I just created a new sub-volume and set the mount point to /scripts - everything is working fine