Created attachment 228841 [details]
dmesg

See attachments for output.

I had a fs of 24 disks in raid6, where I then added another 12 by

[root@btrfs ~]# for f in $(ls /dev/disk/by-path/pci-0000\:*|grep sas|grep -v part|grep -v 500a0b87452d7000|grep -v 500a0b875683e000); do btrfs device add -f "$f" /mnt;done

[root@btrfs ~]# df -H
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs         68G     0   68G   0% /dev
tmpfs            68G  111k   68G   1% /dev/shm
tmpfs            68G  1.8M   68G   1% /run
tmpfs            68G     0   68G   0% /sys/fs/cgroup
/dev/sda1       246G  6.3G  228G   3% /
tmpfs            68G   78k   68G   1% /tmp
tmpfs            14G   33k   14G   1% /run/user/42
tmpfs            14G     0   14G   0% /run/user/0
/dev/sdd         79T   34M   74T   1% /mnt
[root@btrfs ~]# df -H
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs         68G     0   68G   0% /dev
tmpfs            68G  111k   68G   1% /dev/shm
tmpfs            68G  1.7M   68G   1% /run
tmpfs            68G     0   68G   0% /sys/fs/cgroup
/dev/sda1       246G  6.3G  228G   3% /
tmpfs            68G   78k   68G   1% /tmp
tmpfs            14G   33k   14G   1% /run/user/42
tmpfs            14G     0   14G   0% /run/user/0
/dev/sdd        109T   34M  104T   1% /mnt
(reverse-i-search)`': ^C
[root@btrfs ~]# btrfs filesystem balance start /mnt/
WARNING:

	Full balance without filters requested. This operation is very
	intense and takes potentially very long. It is recommended to
	use the balance filters to narrow down the balanced data.
	Use 'btrfs balance start --full-balance' option to skip this
	warning. The operation will start in 10 seconds.
	Use Ctrl-C to stop it.
10 9 8 7 6 5 4 3 2 1
Starting balance without any filters.
ERROR: error during balancing '/mnt/': Read-only file system
There may be more info in syslog - try dmesg | tail
[root@btrfs ~]# dmesg | tail
[337850.868806]  [<ffffffff814c9646>] ? tty_ldisc_deref+0x16/0x20
[337850.868810]  [<ffffffff814c025a>] ? tty_write+0x1ca/0x2f0
[337850.868814]  [<ffffffff814c55a0>] ? n_tty_open+0xd0/0xd0
[337850.868820]  [<ffffffff81363463>] ? selinux_file_ioctl+0x123/0x1e0
[337850.868826]  [<ffffffff8125d4e3>] do_vfs_ioctl+0xa3/0x5d0
[337850.868831]  [<ffffffff8125da89>] SyS_ioctl+0x79/0x90
[337850.868837]  [<ffffffff817e0df2>] entry_SYSCALL_64_fastpath+0x1a/0xa4
[337850.868863] ---[ end trace 0c349ac6c0e12881 ]---
[337850.868869] BTRFS: error (device sdq) in btrfs_create_pending_block_groups:9972: errno=-27 unknown
[337850.869592] BTRFS info (device sdq): forced readonly


# btrfs fi df
Data, RAID6: total=20.41GiB, used=2.88MiB
System, RAID6: total=896.00MiB, used=48.00KiB
Metadata, RAID6: total=1.03GiB, used=160.00KiB
GlobalReserve, single: total=16.00MiB, used=0.00B


# btrfs --version
btrfs-progs v4.7