btrfs send size

[Jim Salter <jim@jrs-s.net>]

Hi list -

Long time ZFS guy here trying to move everything over from ZFS to btrfs, 
which entails a lot of re-scripting and re-learning.

Question of the day: how can I determine the size of a btrfs send 
operation before hand?  I'd like to be able to provide a progress bar 
(I'm accustomed to using pv to do this), but I would need to know a 
rough approximation at least of how much data I'm going to transfer in 
order to be able to do that.

Under ZFS, you can do this:

root@box:~# zfs send -nvi 
data/images@autosnap_2013-11-27_14:00:01_hourly 
data/images@autosnap_2013-11-27_15:00:01_hourly
send from @autosnap_2013-11-27_14:00:01_hourly to 
data/images@autosnap_2013-11-27_15:00:01_hourly estimated size is 42.7M
total estimated size is 42.7M

Which then lets me do this:

root@box:~# zfs send -i data/images@autosnap_2013-11-27_14:00:01_hourly 
data/images@autosnap_2013-11-27_15:00:01_hourly | pv -s 42.7M | ssh 
otherbox zfs receive backup/images

And get a nice progress bar.

I can't find a manpage for the btrfs command that lists ANY information 
about btrfs send, and I haven't found anything online about doing a size 
check on send operations without actually sending the data.  Anybody got 
any help for this?  This is officially a Big Deal for those of us who 
customarily do asynchronous replication, so it would be really, really 
awesome if this could get addressed. =)

Thank you!