btrfs is funny – there is no resize2fs equivalent, but you need to

1) recreate the partitions with e.g. cfdisk

# cfdisk
- select partition
- delete partion
- create new partition on free space
- write (partition table)
- q (cannot re-read partition table)
# reboot
# df -h
/dev/sda3       4,2G  3,6G   63M  99% /

2) tell btrfs to use the partition’s space

# btrfs filesystem resize max /
# df -h
/dev/sda3       9,2G  3,6G  5,1G  42% /

This was a test vm for icinga package builds used locally only – I would not recommend doing that on production systems (nor use btrfs in production).

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