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).