zfs_special_class_metadata_reserve_pct sets a threshold for space in special vdevs to be reserved exclusively for metadata. This prevents small blocks or dedup table from completely consuming a special vdev.
However based on spa_misc.c#L1974 the codepath treats DDT objects the same as regular metadata, so the language in the docs was changed to clarify (emphasis in blockquote only)
zfs_special_class_metadata_reserve_pct sets a threshold for space in special vdevs to be reserved exclusively for metadata. This prevents small data blocks from completely consuming a special vdev.
If the intent is to have DDT obey the reserve percentage then I can make the necessary changes to spa_misc.c instead.
Current language in Module Parameters#zfs-special-class-metadata-reserve-pct states
However based on spa_misc.c#L1974 the codepath treats DDT objects the same as regular metadata, so the language in the docs was changed to clarify (emphasis in blockquote only)
If the intent is to have DDT obey the reserve percentage then I can make the necessary changes to
spa_misc.cinstead.