Whenever somebody asks the question, "How much memory do I need to dedup X terabytes filesystem," the standard answer is "as much as you can afford to buy." This is true and correct, but I don't believe it's the best we can do. Because "as much as you can buy" is a true assessment for memory in *any* situation.
To improve knowledge in this area, I think the question just needs to be asked differently. "How much *extra* memory is required for X terabytes, with dedup enabled versus disabled?" I hope somebody knows more about this than me. I expect the answer will be something like ... The default ZFS block size is 128K. If you have a filesystem with 128G used, that means you are consuming 1,048,576 blocks, each of which must be checksummed. ZFS uses adler32 and sha256, which means 4bytes and 32bytes ... 36 bytes * 1M blocks = an extra 36 Mbytes and some fluff consumed by enabling dedup. I suspect my numbers are off, because 36Mbytes seems impossibly small. But I hope some sort of similar (and more correct) logic will apply. ;-)
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