==== [1] create a valid 4 GB HAMMER image ====
0 bytes transferred in 0.000061 secs (0 bytes/sec)
WARNING: HAMMER filesystems less than 50GB are not recommended!
You may have to run 'hammer prune-everything' and 'hammer reblock'
quite often, even if using a nohistory mount.
==== [2] mount RW briefly so the btree root is allocated ====
rw mount populated and unmounted
==== [3] snapshot btree_root value before patch ====
00000000  00 00 00 21 00 00 00 80                           |...!....|
==== [4] patch vol0_btree_root low 14 bits -> 0x3FFC ====
vol0_btree_root: 0x8000000021000000 -> 0x8000000021003ffc
patched image; btree_root now:
00000000  fc 3f 00 21 00 00 00 80                           |.?.!....|
==== [5] clear dmesg tail marker, then attempt RO mount ====