when you create an sdfs volume, you specify an arbitrary volume size - 8EB must be the maximum size you can specify. I'm guessing the current implementation is only able to deal with 32TB worth of deduplicated chunks - so if you have 8EB of data which you can get a 250000:1 deduplication ratio on, then you could fill up the 8EB volume.

doesn't Luster use a ZFS over Fuse implementation on linux nodes?

anyhow, there are decent alternatives for what ZFS provides (no where near as comprehensive as ZFS, but workable at least). Afaik, there is nothing that provides a deduplicated FS - and if this is able to get 150MB/s then that's a good start.

OpenDedup is actually block level - it just uses a file based backing store to hold the chunks.