Because ZFS is not a content-addressed store (CAS), so block pointer (BP) rewrite is extremely expensive and has not been implemented. And defragmentation requ

Because ZFS is not a content-addressed store (CAS), so block pointer (BP) rewrite is extremely expensive and has not been implemented. And defragmentation requires BP rewrite, or what GP says they do: zfs send, then swap the old and new datasets, but that can't be done atomically and transparently with a script, so that kinda sucks. The fundamental problem is that ZFS is not content addressed. It's almost CAS, but not quite. In ZFS block pointers have physical addresses in the pointer. That means that physical block locations are inexorably part of the ZFS Merkle hash tree. And that means that any change to the location of any block necessitates that the pointers to it must change and so be rewritten, but now you have to find all those pointers -- even if there can be only one pointer, you still have to change it, and when you change it, you change the Merkle hash tree of the dataset. The solution, IMO, is to split all znodes and any interior nodes that have block pointers into two halves. One half should have only logical block pointers free of any physical location pointers, thus the pointers in this half should be some metadata + hashes of the pointed-to blocks. The second half should be a "cache" of physical locations corresponding to the logical block pointers in the first half, plus a hash of just those physical locations. The key is that the Merkle hash tree should not bind physical locations so that changing those locations does not alter the Merkle hash tree. That leaves only the task of updating those second block halves that carry physical locations. So when traversing the tree the filesystem could optimistically read pointed-to blocks from the cached locations, and if the hash of the block read does not match the logical pointer, then go looking in a log of recent block moves. This way a BP rewrite system could simply traverse a dataset looking for blocks to move, copy the blocks to new locations, update the cached locations where the block pointer was found, log the move, then add the old location to the freelist. But ZFS is not like that. ZFS is not a content-addressed store, but it's a Merkle hash tree all the same. That makes BP rewrite insanely too difficult. I do wonder how much effort it would take to make a ZZFS that is based on ZFS but with the CAS redesign sketched above. This is not the first time I've written about this. I think I first proposed this back in the early days of Illumos, and one or two people thought that not hashing the physical locations into the Merkle hash tree would be folly -- I think they're wrong, because either you trust your hash function or you don't. Granted, to make ZFS into a proper CAS does require a strong cryptographic hash function, but ZFS uses one so...