Description of problem:
use_sparse_index, I've tried guessing enough, and no devel test results were provided with rfe 2100608, so as far as I'm concerned, this option doesn't work.

There is no default value:
[root@hayes-01 ~]# lvs -a -o +vdo_use_sparse_index vdo_sanity/vdo_lv
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert VDOSparseIndex
vdo_lv vdo_sanity vwi-a-v--- 100.00g vdo_pool 0.00

[root@hayes-01 ~]# lvconvert --yes --type vdo-pool -n vdo_lv --vdosettings 'vdo_use_sparse_index=1' -V100G vdo_sanity/vdo_pool
Unknown VDO setting "vdo_use_sparse_index".

[root@hayes-01 ~]# lvconvert --yes --type vdo-pool -n vdo_lv --vdosettings 'vdo_sparse_index=1' -V100G vdo_sanity/vdo_pool
WARNING: Converting logical volume vdo_sanity/vdo_pool to VDO pool volume with formating.
THIS WILL DESTROY CONTENT OF LOGICAL VOLUME (filesystem etc.)
Minimum required size for VDO volume: 26130157568 bytes
vdoformat: formatVDO failed on '/dev/vdo_sanity/vdo_pool': VDO Status: Out of space
Command /usr/bin/vdoformat failed.
Cannot format VDO pool volume vdo_sanity/vdo_pool.

[root@hayes-01 ~]# lvconvert --yes --type vdo-pool -n vdo_lv --vdosettings 'vdo_sparse_index=26130157568' -V100G vdo_sanity/vdo_pool
WARNING: Converting logical volume vdo_sanity/vdo_pool to VDO pool volume with formating.
THIS WILL DESTROY CONTENT OF LOGICAL VOLUME (filesystem etc.)
Minimum required size for VDO volume: 26130157568 bytes
vdoformat: formatVDO failed on '/dev/vdo_sanity/vdo_pool': VDO Status: Out of space
Command /usr/bin/vdoformat failed.
Cannot format VDO pool volume vdo_sanity/vdo_pool.

[root@hayes-01 ~]# lvconvert --yes --type vdo-pool -n vdo_lv --vdosettings 'vdo_sparse_index=26130157568b' -V100G vdo_sanity/vdo_pool
Parse error at byte 30 (line 1): unexpected token

[root@hayes-01 ~]# lvconvert --yes --type vdo-pool -n vdo_lv --vdosettings 'vdo_sparse_index=26130157568 bytes' -V100G vdo_sanity/vdo_pool
Parse error at byte 35 (line 1): unexpected token

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