allocate the new extents, after the LV you are mirroring. That is, ifIt looks as though you simply specify the PV from which you want to
you have a logical volume "LV" on /dev/sda1 and you want to mirror it
lvconvert -m1 LV /dev/sdb1
Later I think you would do likewise to remove the local image from
/dev/sda1, leaving the image on /dev/sdb1:
lvconvert -m0 LV /dev/sda1
*I have not yet done this*. Please try it out on a test machine
before risking production data.