Disk extend

[Gokan Atmaca]

Hello

I use KVM. I upgraded to a single disk that was owned by Freebsd with
Qemu. But the disk "ROOT" is still small. What should I do for it ?

root at test:~ # gpart show ada0
=> 34 83886013 ada0 GPT (40G)
34 1024 1 freebsd-boot (512K)
1058 39844864 2 freebsd-ufs (19G)
39845922 2097084 3 freebsd-swap (1.0G)
41943006 41943041 - free - (20G)

I did so;
root at test:~ # gpart resize -i 2 -s 20971520 ada0
gpart: Device busy

How can I do this safely?