raid1: cannot add disk to replace faulty because can only mount fs as read-only.

[Hans Deragon <hans@deragon.biz>]

Greetings,


Warning: Btrfs user here; no knowledge of the inside working of btrfs. 
If I am in the wrong mailing list, please redirect me and accept my 
apologies.

At home, lacking of disks and free SATA ports, I created a raid1 btrfs 
filesystem by converting an existing single btrfs instance into a 
degraded raid1, then added the other driver. The exact commands I used 
have been lost.

Worked well, until one of my drive died. Total death; the OS does not 
detect it anymore. I bought another drive, but alas, I cannot add it:

# btrfs replace start -B 2 /dev/sdd /mnt/brtfs-raid1-b
ERROR: ioctl(DEV_REPLACE_START) failed on "/mnt/brtfs-raid1-b": 
Read-only file system

Here is the command I used to mount it:

mount -t btrfs -o ro,degraded,recovery,nosuid,nodev,nofail,x-gvfs-show 
/dev/disk/by-uuid/975bdbb3-9a9c-4a72-ad67-6cda545fda5e 
/mnt/brtfs-raid1-b

If I remove 'ro' from the option, I cannot get the filesystem mounted 
because of the following error:

BTRFS: missing devices(1) exceeds the limit(0), writeable mount is not 
allowed

So I am stuck. I can only mount the filesystem as read-only, which 
prevents me to add a disk.

It seams related to bug: 
https://bugzilla.kernel.org/show_bug.cgi?id=60594

I am using Ubuntu 16.04 LTS with kernel 4.4.0-59-generic. Is there any 
hope to add a disk? Else, can I recreate a raid1 with only one disk and 
add another, but never suffer from the same problem again? I did not 
lost any data, but I do have some serious downtime because of this. I 
wish that if a drive fail, the btrfs filesystem still mounts rw and 
leave the OS running, but warns the user of a failing disk and easily 
allow the addition of a new drive to reintroduce redundancy. However, 
this scenarios seams impossible with the current state of affair. Am I 
right?


Best regards and thank you for your contribution to the open source 
movement,
Hans Deragon