Dear Anand,
thank you for your help.

On December 10, 2014 at 3:46 AM Anand Jain [off-list ref] wrote:
It depends on the disk that is read first, you could read super block
using btrfs-show-super and check if num_device.

I checked this for all three devices and num_devices is 4 for all of them.
The full output is below.

It may fail if you are mounting more than one subvol during boot and if
you don't have this patch.

commit 0f23ae74f589304bf33233f85737f4fd368549eb
Author: Chris Mason [off-list ref]
Date: Thu Sep 18 07:49:05 2014 -0700

Revert "Btrfs: device_list_add() should not update list when mounted"

This reverts commit b96de000bc8bc9688b3a2abea4332bd57648a49f.

I am running 3.18-rc5+ from linus tree. If I am not mistaken,
 that commit is his tree in since 3.17-rc6.

So I guess the question is: Why does the superblock say
num_devices=4 (and how do I fix it)?
Or am I misunderstanding the problem?

Best regards,
Florian

root@oot:/home/shared# btrfs-show-super /dev/sdb
superblock: bytenr=65536, device=/dev/sdb
---------------------------------------------------------
csum			0x55a3eb1d [match]
bytenr			65536
flags			0x1
magic			_BHRfS_M [match]
fsid			be2b3499-7452-4b91-b664-4ec4d7ff62b9
label			
generation		3003
root			441395970048
sys_array_size		129
chunk_root_generation	3001
root_level		1
chunk_root		442469761024
chunk_root_level	1
log_root		0
log_root_transid	0
log_root_level		0
total_bytes		109927469in the first place8752
bytes_used		160703954944
sectorsize		4096
nodesize		16384
leafsize		16384
stripesize		4096
root_dir		6
num_devices		4
compat_flags		0x0
compat_ro_flags		0x0
incompat_flags		0x61
			( MIXED_BACKREF |
			  BIG_METADATA |
			  EXTENDED_IREF )
csum_type		0
csum_size		4
cache_generation	3003
uuid_tree_generation	3003
dev_item.uuid		5b8c8d20-c330-48a2-814c-c6986b787d52
dev_item.fsid		be2b3499-7452-4b91-b664-4ec4d7ff62b9 [match]
dev_item.type		0
dev_item.total_bytes	500107862016
dev_item.bytes_used	162168569856
dev_item.io_align	4096
dev_item.io_width	4096
dev_item.sector_size	4096
dev_item.devid		2
dev_item.dev_group	0
dev_item.seek_speed	0
dev_item.bandwidth	0
dev_item.generation	0