From the point of view of old clients, segwit adds one coinbase output that contains the root hash of the Merkle tree that commits to the witness transaction ids. It uses 47 extra bytes per block, so technically, yes, it "wastes precious blockchain space". The blockchain on-disk can be pruned though (implemented as an experimental feature in Bitcoin Core 0.11, and with nearly full functionality in 0.12), so calling it "permanently" is not very accurate.
If you're talking about storage space used by segwit-compatible full nodes, well, obviously it will use more space, because it increases block capacity - that capacity has to go somewhere. However:
- The extra space used by witnesses is more prunable than normal block space, as it's not needed by non-validating clients.
- Has less effect on bandwidth, as light clients don't need the witness data.
- Has no effect on the UTXO set, so does not contribute to database growth and/or churn.
- Enables script versions, which will make the introduction of Schnorr signatures much easier later on, which are more space efficient than what we have now (even for simple single-key outputs/inputs).
Let us ignore pruning nodes or anything about SPV nodes. My concern is about full relaying nodes. My assumption is that for a segwit compatible full relaying node to be able to relay the full blockchain it would need to have ALL the data, original blockchain and witness data.
How can ANY of that data be pruned and still be able to be a full relaying node?
If all such data is needed, I want to call the combined size the size of the blockchain. Regardless of whether it is one or 2 different datasets.
So unless there is magic involved that allows relaying data that has been pruned, pruning is IRRELEVANT.
This then gets us to my question that is not being answered. On average, how many bytes in the blockchain will be needed for a standard payment sent via segwit?
Is this ever less than it would be now?
Is this ever the same as it is now?
Is this usually about 50 bytes more per tx?
Unless there is actual savings of blockchain space, then it would be a failure as far as reducing blockchain usage. What am I missing?
James