I forget the exact names, but go with it. :-) ZFS has two tables: A Block Pointer Table. This is your hashes of blocks and stays in RAM to let you know where o

I forget the exact names, but go with it. :-) ZFS has two tables: A Block Pointer Table. This is your hashes of blocks and stays in RAM to let you know where on disk you'll find a particular block. It's also referenced during writing to ensure that the COW operations are transactional. The DDT (DataDedupeTable) is similar, but points to de-duped blocks. You need enough RAM to keep this in memory at all times. Otherwise for every write you'll have to scan through the entire table on disk, find matching blocks, then modify it. You'll also have to scan through it for every read to see if the block you're looking for is de-duped. I've probably mixed up the details. I haven't read the source, just used it. But that's my (maybe flawed) understanding of how it works. Bottom line though, you DO need enough RAM to keep the DDT in memory. If you don't you'll see SEVERE thrashing and SSD or no SSD your disk performance will slow down from thousands of IOPS to tens of IOPS or lower. This exact thing happened to us with Sun's 7310, loaded with the Sun specified SSDs. You need to be very sure you have the hardware for de-dupe, and even then the payoff is so small... The only real use-case I can think of is if you're running a VPS.