Perfect!! Just what I was looking for.
Sorry for the delay, because before doing so, I preferred to test to see if it 
actually worked.

I have a doubt. The system works perfectly, but at the time of deleting the 
writing disk and merging the data on the read-only disk I fail to understand 
the process.

I have tried to remove the seed bit on disk A and delete the write B as you 
mention, and so move the data to A, but tells me that disk B does not exist.
These are the orders I have made:

md127-> A
md126-> B

btrfstune -S 0 /dev /md127
mount /dev/md127 /mnt (I mount this disk since the md126 gives error)
btrfs device delete /dev/md126 /mnt
ERROR: error removing device '/dev/md126': No such file or directory

Another thing I've tried is to remove disk B without removing the seed bit, 
but it gives me the error:

ERROR: error removing device '/dev/md126': unable to remove the only writeable 
device.

Any ideas about it?
Thank you very much for the reply.
Greetings.

El martes, 12 de septiembre de 2017 6:34:15 (CEST) Andrei Borzenkov escribió:
> 11.09.2017 21:17, Senén Vidal Blanco пишет:
> > I am trying to implement a system that stores the data in a unit (A) with
> > BTRFS format that is untouchable and that future files and folders created
> > or modified are stored in another physical unit (B) with BTRFS format.
> > Each year the new files will be moved to store A and start over.
> > 
> > The idea is that a duplicate of disk A can be made to keep it in a safe
> > place and that the files stored there can not be modified until the
> > mixture of (A) and (B) is made.
> 
> This can probably be achieved using seed device. Mark original device as
> seed and all changes will go to another writable device, similar to
> overlay; then remove seed bit from original device, "btrfs device remove
> writable" device and it should relocate its content back. Rinse and repeat.

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