lvconvert fails with: Insufficient free space: 1 extents needed, but only 0 available

Issue

• lvconvert fails with: Insufficient free space: 1 extents needed, but only 0 available when creating mirror with same disk size:

# fdisk -l

Disk /dev/vda: 10.7 GB, 10737418240 bytes, 20971520 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x00038b2a

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1   *        2048     1026047      512000   83  Linux
/dev/vda2         1026048    20971519     9972736   8e  Linux LVM            <===

Disk /dev/vdb: 10.7 GB, 10737418240 bytes, 20971520 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x5c2e93ac

   Device Boot      Start         End      Blocks   Id  System
/dev/vdb1            2048     1026047      512000   8e  Linux LVM
/dev/vdb2         1026048    20971519     9972736   8e  Linux LVM            <===