Re: Which device is missing ?

[Pierre Couderc <pierre@couderc.eu>]

On 10/08/2018 06:14 PM, Hugo Mills wrote:
> On Mon, Oct 08, 2018 at 04:10:55PM +0000, Hugo Mills wrote:
>> On Mon, Oct 08, 2018 at 03:49:53PM +0200, Pierre Couderc wrote:
>>> I ma trying to make a "RAID1" with /dev/sda2 ans /dev/sdb (or similar).
>>>
>>> But I have stranges status or errors  about "missing devices" and I
>>> do not understand the current situation :
>>>
>>>
>>> root@server:~# btrfs fi show
>>> Label: none  uuid: 28c2b7ab-631c-40a3-bab7-00dac5dd20eb
>>>          Total devices 1 FS bytes used 190.91GiB
>>>          devid    1 size 1.82TiB used 196.02GiB path /dev/sda2
>>>
>>> warning, device 1 is missing
>>> Label: none  uuid: 2d45149a-fb97-4c2a-bae2-4cfe4e01a8aa
>>>          Total devices 2 FS bytes used 116.18GiB
>>>          devid    2 size 1.82TiB used 118.03GiB path /dev/sdb
>>>          *** Some devices missing
>>     This looks like you've created a RAID-1 array with /dev/sda2 and
>> /dev/sdb, and then run mkfs.btrfs again on /dev/sda2, overwriting the
>> original [part of a] filesystem on /dev/sda2, and replacing it with a
>> wholly different filesystem. Since the new FS on /dev/sda2 (UUID
>> 28c2...) doesn't have the same UUID as the original FS (UUID 2d45...),
>> and the original FS was made of two devices, btrfs fi show is telling
>> you that there's some devices missing -- /dev/sda2 is no longer part
>> of that FS, and is therefore a missing device.
>>
>>     I note that you've got data on both filesystems, so they must both
>> have been mounted somewhere and had stuff put on them.
>>
>>     I recommend doing something like this:
>>
>> # mkfs /media/btrfs/myraid1 /media/btrfs/tmp
>> # mount /dev/sdb /media/btrfs/myraid1/
>> # mount /dev/sda2 /media/btrfs/tmp/              # mount both filesystems
>> # cp /media/btrfs/tmp/* /media/btrfs/myraid1     # put it where you want it
>> # umount /media/btrfs/tmp/
>> # wipefs /dev/sda2                               # destroy the FS on sda2
>> # btrfs replace start 1 /dev/sda2 /media/btrfs/myraid1/
>>
>>     This will copy all the data from the filesystem on /dev/sda2 into
>> the filesystem on /dev/sdb, destroy the FS on sda2, and then use sda2
>> as the second device for the main FS.
>>
>> *WARNING!*
>>
>>     Note that, since the main FS is missing a device, it will probably
>> need to be mounted in degraded mode (-o degraded), and that on kernels
>> earlier than (IIRC) 4.14, this can only be done *once* without the FS
>> becoming more or less permanently read-only. On recent kernels, it
>> _should_ be OK.
>>
>> *WARNING ENDS*
>     Oh, and for the record, to make a RAID-1 filesystem from scratch,
> you simply need this:
>
> # mkfs.btrfs -m raid1 -d raid1 /dev/sda2 /dev/sdb
>
>     You do not need to run mkfs.btrfs on each device separately.
>
>     Hugo.
Thnk you very much. I understand a bit better. I think  that I have 
nothing of interest on /dev/sdb and that its contents is the result of 
previous trials.
And that my system is on /dev/dsda2 as :

root@server:~# df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            3.9G     0  3.9G   0% /dev
tmpfs           787M  8.8M  778M   2% /run
/dev/sda2       1.9T  193G  1.7T  11% /
tmpfs           3.9G     0  3.9G   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           3.9G     0  3.9G   0% /sys/fs/cgroup
/dev/sda1       511M  5.7M  506M   2% /boot/efi
tmpfs           100K     0  100K   0% /var/lib/lxd/shmounts
tmpfs           100K     0  100K   0% /var/lib/lxd/devlxd
root@server:~#

Is it exact ?
If yes, I suppose I should wipe data on /dev/sdb, then build the RAID by 
expanding /dev/sda2.
So I should :

wipefs /dev/sdb
btrfs device add /dev/sdb /
btrfs balance start -v -mconvert=raid1 -dconvert=raid1 /


Does it sound correct ? (my kernel is boot/vmlinuz-4.18.0-1-amd64)