Odd behavior of subvolume find-new

I've been creating some time-based snapshots, e.g.

     # btrfs subvolume snapshot @root 2012-01-09-@root

After some changes, I wanted to see what had changed, so I tried:

     # btrfs subvolume find-new @root 2012-01-09-@root
     transid marker was 37

which doesn't print anything out. Curiously, if I make a snapshot of the snapshot, then I get output from the delta:

     # btrfs subvolume snapshot 2012-01-09-@root tmp
     # btrfs subvolume find-new @root tmp
     ..... lots of output .....

I haven't seen this behavior on other filesystems or subvolumes.

My intent was to filter through the small script below to compute the size of the delta.

Thanks,
David

#! /usr/bin/perl

# Process the output of btrfs subvolume find-new and print out the

# size used by the new data.  Doesn't show delta in metadata, only the
# data itself.
use strict;

my $bytes = 0;

while (<>) {
	if (/ len (\d+) /) {
		$bytes += $1;
	}
}

printf "%d bytes\n", $bytes;
printf "%.1f MByte\n", $bytes / 1024.0 / 1024.0; printf "%.1f GByte\n", $bytes / 1024.0 / 1024.0 / 1024.0;