Comment 1 for bug 1477858

Meanwhile I think this is a more deeper Linux bug, because...

This server has (besides some Seagates) 2 Hitachi (HGST) SATA disks, but
Linux shows 3 of them with 2 with the same serial number:

root@toy02:~# lsblk -o NAME,SIZE,FSTYPE,LABEL,MOUNTPOINT
NAME SIZE FSTYPE LABEL MOUNTPOINT
sda 1.8T drbd
sdb 1.8T btrfs data
sdc 1.8T drbd
sdd 1.8T drbd
sde 1.8T drbd
sdf 68G
|-sdf1 14.9G ext4 /
|-sdf2 14.9G ext4
|-sdf3 14.9G swap swap [SWAP]
`-sdf4 23.3G ext4 tmp /tmp
sr0 1024M
drbd0 1.8T btrfs data
drbd1 1.8T
drbd2 1.8T btrfs data
drbd3 1.8T btrfs data

root@toy02:~# hdparm -I /dev/sdb| grep Number:
        Model Number: HGST HUS724020ALA640
        Serial Number: PN2134P5G2P2AX

root@toy02:~# hdparm -I /dev/sdd| grep Number:
        Model Number: HGST HUS724020ALA640
        Serial Number: PN2134P5G2P2XX

root@toy02:~# hdparm -I /dev/sde| grep Number:
        Model Number: HGST HUS724020ALA640
        Serial Number: PN2134P5G2P2AX

This is why btrfs gets confused:

root@toy02:~# btrfs_info
/data :

Data, RAID0: total=108.00GiB, used=107.55GiB
System, RAID1: total=32.00MiB, used=16.00KiB
Metadata, RAID1: total=1.00GiB, used=139.59MiB

Label: data uuid: 411af13f-6cae-4f03-99dc-5941acb3135b
        Total devices 3 FS bytes used 107.68GiB
        devid 1 size 1.82TiB used 37.03GiB path /dev/drbd0
        devid 3 size 1.82TiB used 36.03GiB path /dev/drbd2
        devid 4 size 1.82TiB used 37.00GiB path /dev/sdb

/dev/sdb is WRONG, it should be /dev/drbd3, because I have expanded the
btrfs filesystem with:

root@toy02:~# btrfs device add -f /dev/drbd3 /data

More info:

root@toy02:~# lf drbd /dev
dRWX - 2015-08-10 14:17 /dev/drbd
lRWX - 2015-08-10 14:18 /dev/block/147:0 -> ../drbd0
lRWX - 2015-08-11 14:45 /dev/block/147:1 -> ../drbd1
lRWX - 2015-08-11 14:00 /dev/block/147:2 -> ../drbd2
lRWX - 2015-08-11 14:00 /dev/block/147:3 -> ../drbd3
lRWX - 2015-08-11 14:00 /dev/drbd/by-disk/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2AX -> ../../../../drbd3
lRWX - 2015-08-11 14:00 /dev/drbd/by-disk/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2XX -> ../../../../drbd2
lRWX - 2015-08-11 14:45 /dev/drbd/by-disk/disk/by-id/ata-ST2000DM001-1CH164_W241L86K -> ../../../../drbd1
lRWX - 2015-08-10 14:18 /dev/drbd/by-disk/disk/by-id/ata-ST2000DM001-1CH164_W241L88L -> ../../../../drbd0
lRWX - 2015-08-11 14:00 /dev/drbd/by-res/d2 -> ../../drbd2
lRWX - 2015-08-11 14:00 /dev/drbd/by-res/d3 -> ../../drbd3
lRWX - 2015-08-10 14:18 /dev/drbd/by-res/data/0 -> ../../../drbd0
lRWX - 2015-08-11 14:45 /dev/drbd/by-res/data/1 -> ../../../drbd1
bRW- 147,000 2015-08-10 14:18 /dev/drbd0
bRW- 147,001 2015-08-11 14:45 /dev/drbd1
bRW- 147,002 2015-08-11 14:00 /dev/drbd2
bRW- 147,003 2015-08-11 14:00 /dev/drbd3

root@toy02:~# lf HGST /dev
lRWX - 2015-08-11 16:41 /dev/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2AX -> ../../sdb
lRWX - 2015-08-10 16:45 /dev/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2XX -> ../../sdd
lRWX - 2015-08-11 14:00 /dev/drbd/by-disk/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2AX -> ../../../../drbd3
lRWX - 2015-08-11 14:00 /dev/drbd/by-disk/disk/by-id/ata-HGST_HUS724020ALA640_PN2134P5G2P2XX -> ../../../../drbd2

root@toy02:~# lf sde /dev
lRWX - 2015-08-10 16:45 /dev/block/8:64 -> ../sde
lRWX - 2015-08-10 16:45 /dev/disk/by-path/pci-0000:08:00.0-scsi-0:1:2:0 -> ../../sde
bRW- 008,064 2015-08-10 16:45 /dev/sde