back&forth send/receiving?

[Christoph Anton Mitterer <calestyo@scientia.org>]

Hey.

I've had asked[0] this a while ago already and back then the conclusion
seems to have been that it wasn't (yet) possible but could in principle
be done.

The idea was that one main usage of btrfs' send/receive is probably
making incremental backups.

So one might have a setup, where a master is send/received to multiple
copies of that like in:

master-+-> copy1
       |-> copy2
       \-> copy3

with those all being different storage mediums.

At some point, master's HDD may e.g. break or at least reach an age
where one wants to replace it with a new one.
So either one would make e.g. copy1 the new master, or buy new HDD for
that, while keeping all the other copyN.

Problem is now, how to best continue with the above schema?

Either one just uses dd and copy over the old master to the new one
(keeping the very same btrfs).
This of course doesn't work if the old master really broke. And even if
it didn't it may be impractical (consider e.g. a 20 TB HDD which is
only half filled - why copying 10TB of empty space).


What one would IMO want is, that send/receive also works like that:
a) copy1/new-master-+-> copy2
                    |-> copy3
                    \-> copy4 (new)

b) once:
   copy1 ---> new-master

   following that: 
   new-master-+-> copy1
              |-> copy2
              \-> copy3


Did anything change with respect to v2 send/receive? Or would it be
possible to get such feature sooner or later? :-)


Thanks,
Chris.



[0] https://lore.kernel.org/linux-btrfs/157ed91bb66820d1fef89eb05d00e65c25607938.camel@scientia.net/