Request for comments about support btrfs in lvresize

Hi
 I am drafting patches for btrfs support in lvresize command. All things =
went smoothly until I realized
multi-devices btrfs handling is a little complicated.

For one device btrfs, fslastblock * fsblocksize/FSSIZE is the correct =
value like ext* and xfs
But for multi-devices btrfs, the two values are whole fs size. The used =
size of device can be
known by btrfs-progs or superblock read and parse.

Heming suggested that maybe in fs_get_info(), execute of a external =
script named=20
lvresize_btrfs_helper.sh which calls btrfs-progs to gather information =
is a feasible way.
We both admit it=E2=80=99s not a grace way so here seek advices from =
you. Thanks.

=E2=80=94=20
Su