The documentation here "https://docs.microsoft.com/en-us/windows-server/storage/data-deduplication/interop" is not clear to me because the reference of an unsupported configuration of deduplication on volumes with multiple tiers is in a bullet point that references NTFS-formatted volumes
Data Deduplication is fully supported on Storage Spaces Direct NTFS-formatted volumes (mirror or parity). Deduplication is not supported on volumes with multiple tiers. See Data Deduplication on ReFS for more information.Blockquote
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However, according to this "https://techcommunity.microsoft.com/t5/windows-server-ama/refs-s2d-and-dedup-compression/m-p/100456" in Windows Server version 1709 deduplication introduced a feature known as near-inline optimization, which works (specifically) with ReFS mirror-accelerated parity. Unless this feature was deprecated before Windows server 2019, I would expect it to be supported on ReFS mirror-accelerated parity.
Mirror-accelerated parity is appealing as Cosmos points out here "https://techcommunity.microsoft.com/t5/storage-at-microsoft/here-s-what-you-missed-8211-five-big-announcements-for-storage/ba-p/428257".
