Hi,

lvextend doesn't allocate extents appropriately if the LV is mirrored.
Attached patch fixes the problem.
If this looks reasonable, please consider to include in LVM2.

Details of the problem:

Suppose I have 4 PVs with sufficient free space and a 2-sided
mirrored LV with size=4MB, I would expect the following LV
as a result of "lvextend -L+4MB":

# lvs -a -olv_name,seg_size,devices vg4
  LV             SSize Devices
  lv0            8.00M lv0_mimage_0(0),lv0_mimage_1(0)
  [lv0_mimage_0] 8.00M /dev/sda(0)
  [lv0_mimage_1] 8.00M /dev/sdb(0)
  [lv0_mlog]     4.00M /dev/sdc(0)

However, actual result was following:

# lvs -a -olv_name,seg_size,devices vg4
  LV             SSize Devices
  lv0            8.00M lv0_mimage_0(0),lv0_mimage_1(0)
  [lv0_mimage_0] 4.00M /dev/sda(0)
  [lv0_mimage_0] 4.00M /dev/sdd(0)
  [lv0_mimage_1] 4.00M /dev/sdb(0)
  [lv0_mimage_1] 4.00M /dev/sda(1)
  [lv0_mlog]     4.00M /dev/sdc(0)

In this case, failure of a PV (/dev/sda) would break whole LV (lv0),
which is not what we expect from mirrored LV.

The cause of the problem is that _check_contiguous() doesn't care
layered LV and cannot find out contiguousness between PV areas.

Thanks,
-- 
Jun'ichi Nomura, NEC Corporation of America