[linux-lvm] Strange errors trying to mirror

[Phillip Susi <psusi cfl rr com>]

I'm trying to mirror my root fs and get errors that make no sense.  What
am I missing?

  PV         VG   Fmt  Attr PSize   PFree
  /dev/sda1  vg0  lvm2 a-   111.79G  61.79G
  /dev/sdb1  vg0  lvm2 a-   111.79G 111.79G

lvconvert -m1 -i 5 vg0/root
  Insufficient suitable allocatable extents for logical volume : 7680
more required
  Unable to allocate extents for mirror(s).

Then I tried explicitly telling it to allocate from sdb1:

lvconvert -m1 -i 5 vg0/root /dev/sdb1
  Not enough PVs with free space available for parallel allocation.
  Consider --alloc anywhere if desperate.
  Unable to allocate extents for mirror(s).

There are two PVs so why does it complain?