root@debian8:~# umount /mnt/ext4fs 
root@debian8:~# lvreduce -L -10G -r /dev/vg00/ext4fs
fsck from util-linux 2.25.2
/dev/mapper/vg00-ext4fs: 34367/4587520 files (0.0% non-contiguous), 519181/18350080 blocks
resize2fs 1.42.12 (29-Aug-2014)
Resizing the filesystem on /dev/mapper/vg00-ext4fs to 15728640 (4k) blocks.
The filesystem on /dev/mapper/vg00-ext4fs is now 15728640 (4k) blocks long.

  Size of logical volume vg00/ext4fs changed from 70.00 GiB (17920 extents) to 60.00 GiB (15360 extents).
  Logical volume ext4fs successfully resized