Can't mount BTRFS volume using fstab

I installed Alpine on /dev/sda in sys mode using ext4. Then I created a btrfs volume on /dev/sdb and /dev/sdc using:

apk add btrfs-progs
modprobe btrfs && echo btrfs > /etc/modules
mkfs.btrfs -m raid1 -d raid1 /dev/sdb /dev/sdc
mkdir /data && mount /dev/sdb /data

This is working fine. Now I add this mount to /etc/fstab like this:

UUID=da110dca-aed5-48b8-a5b6-c1b41c10c419 /data btrfs defaults 0 0

On reboot I get the following error:

mount: mounting /dev/sdc on /data failed: Invalid argument

Output of dmesg:

[    5.950849] BTRFS: device fsid da110dca-aed5-48b8-a5b6-c1b41c10c419 devid 2 transid 12 /dev/sdc
[    5.952786] BTRFS info (device sdc): disk space caching is enabled
[    5.952791] BTRFS info (device sdc): has skinny extents
[    5.954055] BTRFS warning (device sdc): devid 1 uuid 6a45e7d4-78df-4263-be09-b83a3a15e6e0 is missing
[    5.954063] BTRFS error (device sdc): failed to read the system array: -5
[    6.002364] BTRFS error (device sdc): open_ctree failed

I fixed this by adding /sbin/btrfs device scan to the top of the start method in /etc/init.d/localmount.

However, I have a feeling that this command is already ran on boot but only after mounting /etc/fstab, because when I simply run mount /dev/sdb /data right after booting it’s working.

It seems that this command is being called if the root file system is btrfs (see #6903); I think this should also be done if there is any other btrfs file system in /etc/fstab.

(from redmine: issue id 9539, created on 2018-10-08)