Balance using 1 cpu core

Hello,
I’m currently changing from raid 5 to raid 6 and noticed the balance is taking very long time (will take ~4 days).
I thought this was odd and there could be potential that a disk will fail in the middle of the balance operation and then the raid is dead.
Did some digging to see why this is taking so long and noticed after installing “htop” that only one cpu core is used and it’s at 100% all the time.

Is this a known limitation?

Htop graph:
[root@hop ~]# htop

1 [| 0.7%] Tasks: 47, 24 thr; 2 running
2 [ 0.0%] Load average: 3.10 2.58 1.97
3 [||||||||||||||||||||||||||||||||||||||||||||||||||||||100.0%] Uptime: 00:37:12
4 [|| 2.0%]
Mem[||||||||||||||||||||||||||||| 897M/5.82G]
Swp[ 0K/2.98G]

PID USER PRI NI VIRT RES SHR S CPU% MEM% TIME+ Command
4421 root 20 0 119M 3996 2988 R 1.3 0.1 0:05.64 htop
3551 root 20 0 219M 22432 7252 S 0.0 0.4 0:02.84 /usr/bin/python /opt/rockstor/bin/supervisord -c /opt/rockstor/etc/supervisord

@Fredrik A belated welcome to the Rockstor community to you.

Yes it is a know problem, there are multiple reports of such on the linux-btrfs mailing list.

Also note that the btrfs parity levels ie raid5 and raid6 are not recommended for production use.