That’s not the aspect of the deduplication process that I was referring to. Btrfs only supports offline deduplication, where you periodically run a process tha

That’s not the aspect of the deduplication process that I was referring to. Btrfs only supports offline deduplication, where you periodically run a process that searches for duplicate blocks and combines them. Meanwhile ZFS supports online deduplication, where every block written is checked against an index to see if it is a duplicate of some existing block. That index is what takes up the memory that jcalvinowens complained about. When deduplication is turned on, ZFS keeps that whole index in memory all of the time. Meanwhile btrfs only needs that index to be loaded while the deduplicator is running, so it doesn’t show up most of the time. Personally I don’t think that either is really worth the cost; the benefit is so small that the cost isn’t worth paying.