Description of problem:
When VDO is created, some space given to it is taken by index and the rest is divided between slabs and the remaining space is left unused and unusable. Here is an example when creating VDO on 6G device:

# vdo create --name vdo --device 6G_device
Because default VDO slab size is 2G and default index size is around 2.5G, the actual usable space for user data is only single slab of 2G (6 - 2.5 = 3.5; 3.5 % 2 = 1)
# vdo status | grep 'Physical size'
    Physical size: 6G
# vdo status | grep 'Logical size'
    Logical size: 6G
# lsblk
NAME                    MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
--snip--
sdc                       8:32   0  5.5T  0 disk
└─vg-lv                 253:2    0    6G  0 lvm
  └─vdo                 253:3    0    6G  0 dm
Creating a filesystem and mounting it to check 'df':
# df -h
Filesystem                           Size  Used Avail Use% Mounted on
--snip--
/dev/mapper/vdo                       6G   33M   14G   1% /root/vdo

Every obvious value at this point reports the VDO size to be 6G, but the actual size is only 2G:
# vdo status | grep 'slab count'
        slab count: 1
# vdo status | grep 'Slab size'
    Slab size: 2G

Version-Release number of selected component (if applicable):
vdo-6.1.0.55-9

How reproducible:
100%

Steps to Reproduce:
1. vdo create --name vdo --device device
2. # vdo status | grep 'Physical size'
should equal
# vdo status | grep 'slab count'
        slab count: 1
times
# vdo status | grep 'Slab size'
    Slab size: 2G


Actual results:
6G vs 2G

Expected results:
2G vs 2G

Additional info: