Unexpected behavior involving file attributes and snapshots.

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

I was just experimenting with snapshots on 4.9.0, and came across some 
unexpected behavior.

The simple explanation is that if you snapshot a subvolume, any files in 
the subvolume that have the NOCOW attribute will not have that attribute 
in the snapshot.  Some further testing indicates that this is the only 
file attribute that isn't preserved (I checked all the chattr flags that 
BTRFS supports).

I'm kind of curious whether:
1. This is actually documented somewhere, as it's somewhat unexpected 
given that everything else is preserved when snapshotting.
2. This is intended behavior, or just happens to be a side effect of the 
implementation.