[linux-lvm] show free space _inside_ logical volume

[Gabriel <jarod125@gmail.com>]

Hi all,

I'm interested to know if it is possible to check the amount of free
space _inside_ a LV. I know that usually a LV is fully "occupied" by a
partition, but there is a (short) period when there is a gap between
the size of the partition and the size of the LV it resides on. That
happens _after_ doing a lvextend and _before_ doing the resize
(expand) of the filesystem.

First thing I thought of was lvdisplay. While I can see how many LEs
the LV has, I cannot see how many of them are in use by the filesystem
and how many are free (the way pvdisplay shows total/free/allocated
PEs).

It was suggested to me that I try some filesystem tools (resize2fs,
dumpe2fs, tune2fs etc.) that might show me the free space. resize2fs
does not have a 'dry-run' option that would have shown the new size
(in blocks) of the filesystem and I didn't find anything useful in
tune2fs' manual either.

dumpe2fs shows various information such as block count, free blocks
and block size, but I see no modifications when running this before
and after the lvextend command.

I also tried parted -l, but to no avail.