Re: trouble replacing second disk from pair

[Roman Mamedov <rm@romanrm.net>]

On Wed, 8 Dec 2021 12:40:14 +0000
Rory Campbell-Lange <rory@campbell-lange.net> wrote:

> We're trying to upgrade the disks in a btrfs pair, and I have successfully replaced one of them using btrfs replace. I presently have 
> 
> Label: 'btrfs-bkp'  uuid: da90602a-b98e-4f0b-959a-ce431ac0cdfa
> 	Total devices 2 FS bytes used 700.29GiB
> 	devid    2 size 2.73TiB used 1.73TiB path /dev/mapper/cdisk4
> 	devid    3 size 2.73TiB used 1.75TiB path /dev/mapper/cdisk2
> 
> I'd like to get rid of cdisk2 and replace it with a new disk.
> 
> However I'm unable to mount cdisk4 (the new disk) in degraded mode to allow me to similarly replace cdisk2 as I previously did for cdisk3. Is this because some of the data in only on cdisk2? If so I'd be grateful to 
> know how to ensure the two disks have the same data and to allow cdisk2 to be replaced.

Looks like you need to ensure everything is RAID1 first:

  btrfs balance start -dconvert=raid1,soft /bkp
  btrfs balance start -mconvert=raid1,soft /bkp
  btrfs balance start -sconvert=raid1,soft /bkp

It might warn you about operating on system chunks, but I believe this still
needs to be done. 

If still unable to mount after that, then post what mount command do you use
and which messages you get in dmesg.

-- 
With respect,
Roman