In a botched attempt to move my root partition from one disk to another I have done the following:

• added new disk
• partitioned it with parted (part #3 is now almost totally filling the disk)

• initialized a physical volume

  $ pvcreate /dev/sdb3
  Physical volume "/dev/sdb3" successfully created
  

• extended the volume group to include this new physical disk

  $ vgextend myvg /dev/sdb3
  Volume group "myvg" successfully extended
  

• extended the logical volume (I think this is where I ballsed it up: I think I should have pvmove'ed stuff to the new pv in that group - can someone confirm?)

  $ lvextend /dev/mapper/myvg-root /dev/sdb3
  

I would now like to undo the lvextend and then proceed with the original plan of moving the content of the old physical volume over to the new physical volume. Can I reduce the logical volume (I have not yet touched the ext4 partition that sits in /dev/mapper/myvg-root with something like resizefs) without fear of damaging the ext4 filesystem? If so, how do I tell it to reduce by exactly the right amount?

$ lvreduce --by-exactly-the-amount-occupied-by-PV /ev/sdb3 /dev/mapper/myvg-root