- Boot from a helper system and get a root shell (I used the rescue mode of the Debian installer)
- Check the filesystem of the partition to resize:
e2fsck -f /dev/vg_foo/lv_bar
- Resize the filesystem (make it a bit smaller than the target size, to have a safety margin when resizing the logical volume):
resize2fs /dev/vg_foo/lv_bar 180G
- Reduce size of the logical volume:
lvreduce -L 190G /dev/vg_foo/lv_bar
- Grow the filesystem to the new size of the logical volume:
resize2fs /dev/vg_foo/lv_bar
- For good measure run another filesystem check:
e2fsck -f /dev/vg_foo/lv_bar