Hello, the inline zero-block deduplication is automatically enabled when dedupe is enabled, since Clustered Data ONTAP 8.3.2.
Inline detection on deduplication-enabled volumes to identify blocks of zeros
Starting with Data ONTAP 8.3, deduplication performs inline detection on volumes to identify blocks of zeros and makes those blocks available for data to be rewritten, resulting in improved volume performance.
It does not need to be inline-dedupe just enabling post-process dedupe on the HDD only systems is sufficient to enable this feature (volume efficiency on). It is not configurable and is automatically enabled when dedupe is also enabled. To confirm the saving being realised use the command: