# check / repair

    sudo btrfs check /dev/sdc1
    sudo btrfs check --repair /dev/sdc1
    sudo btrfs check --clear-space-cache v1|v2 /dev/sdc1


# subvolume

    btrfs subvolume create current

    btrfs subvolume snapshot    current 2018-01-04
    btrfs subvolume snapshot -r current 2018-01-04  # read only

    sudo btrfs subvolume list .
    # restrict list to `dir`, given that `dir` is a subvolume on its own
    sudo btrfs subvolume list -o dir/

    sudo btrfs subvolume delete 2018-01-04

    # set / unset readonly
    btrfs property set -ts subvol ro false


# send/receive

- src needs to be subvolume
- have to create readonly snapshot of src before syncing
- `send` compares two readonly snapshots, need to keep old one around
- `send` seems to be root, `receive` also. confirm?

    # init
    btrfs subvolume create src
    btrfs subvolume snapshot -r src src-`date -Iminutes`
    sudo btrfs send src-2018-01-18T14\:52+01\:00 | sudo btrfs receive backup/
    # increment
    btrfs subvolume snapshot -r src src-`date -Iminutes`
    sudo btrfs send -p src-2018-01-18T14\:52+01\:00 src-2018-01-18T14\:57+01\:00 | sudo btrfs receive backup/


# raid

    btrfs filesystem show
    btrfs filesystem df /mnt/
    sudo mkfs.btrfs /dev/sda1 -L label
    sudo btrfs device add /dev/sda2 /mnt/
    sudo btrfs balance start -dconvert=raid1 -mconvert=raid1 /mnt/
    sudo btrfs device delete /dev/sda1 /mnt/
    sudo btrfs balance pause /mnt/

    sudo btrfs balance start -f -mconvert=single -dconvert=single /mountpoint
    sudo btrfs device remove missing /mountpoint
    sudo btrfs balance start -mconvert=dup /mountpoint