[root@mirror ~]# df -h
Filesystem      Size  Used Avail Use% Mounted on
dev             955M     0  955M   0% /dev
run             963M  596K  962M   1% /run
/dev/sda2        20G  2.1G   17G  11% /
tmpfs           963M     0  963M   0% /dev/shm
tmpfs           963M     0  963M   0% /tmp
/dev/sdb        125G  105G   19G  85% /srv/ftp
tmpfs           193M  4.0K  193M   1% /run/user/0
[root@mirror ~]# btrfs filesystem resize max /srv/ftp
Resize device id 1 (/dev/sdb) from 125.00GiB to max
[root@mirror ~]# df -h
Filesystem      Size  Used Avail Use% Mounted on
dev             955M     0  955M   0% /dev
run             963M  596K  962M   1% /run
/dev/sda2        20G  2.1G   17G  11% /
tmpfs           963M     0  963M   0% /dev/shm
tmpfs           963M     0  963M   0% /tmp
/dev/sdb        200G  105G   94G  53% /srv/ftp
tmpfs           193M  4.0K  193M   1% /run/user/0