fully balance before shrink

when looking at a heatmap of the partition it's still not quite ideal,
when I manually shrink it I get a better block layout, so let's try to
balance the blocks before shrinking. if nothing else it should mean less
relocation during shrinking

btrfs-send-receive.sh

@@ -57,6 +57,9 @@ btrfs filesystem usage .
 duperemove -dr . > /dev/null
 btrfs subvolume sync .
 btrfs filesystem sync .
+btrfs balance start --full-balance --enqueue .
+btrfs subvolume sync .
+btrfs filesystem sync .
 ## And to finish things off we shrink the filesystem to the minimum size.
 $OUTPUT_DIR/btrfs-shrink.py
 ## Sync changes to disk.

btrfs-shrink.py

@@ -8,6 +8,11 @@
 # portion but leave a bit of a buffer behind. Then we keep resizing until the
 # resize starts failing.
 
+# TODO we can produce an even more squeezed image by deriving a complete partition
+# from the initial partition. https://btrfs.readthedocs.io/en/latest/Seeding-device.html
+# It essentially transfers the data from the seed device with nary a fragmentation.
+# Requires juggling loop devices though, and the gains are in the sub 500MiB range.
+
 import json
 import os
 import math