N + 2*numtxids + numvins > N
I still claim that is true, not sure how that loses me any credibility
I believe I have forgotten to address this. Can you please explain how you are getting this?
AFAIK the txids aren't in any structure used by Bitcoin except in the inventories. Those might be stored, depends on the implementation. However, when it comes to the wtxids, there is absolutely no reason to store them. Their sole purpose is to simply have a hash of all of the data in a segwit transaction and have that be applied to the witness root hash in the coinbase transaction. There is no need to store the wtxids since nothing ever references them.
Where are you getting numvins from?
Anyways, your formula is wrong if you assume that the regular txid is currently being stored. Rather it should be
N + wtxid + numvins > N
and that is only if you are going to store wtxids which are not necessary to store anyways.
I was told the extra space needed was 2 bytes per segwit tx plus 1 byte per vin, though maybe the 1 byte per vin can be reduced to 1 bit. Not sure how that is possible without new script opcodes, so maybe that is a possibility in the fullness of time sort of thing.
Regardless, the total space needed is more for segwit tx than normal tx, this is confirmed by wuille, lukejr and gmaxwell.
now I never said segwit wasnt impressive tech as that is quite a small overhead. my point is that segwit does not reduce the permanent space needed and if you feel that the HDD space needed to store the blockchain (or the data need to be shared between full nodes) is a factor that is important to scalability, then segwit does not help scalability regarding those two factors.
I do not speak about any other factors, only the permanent space used. Originally I was told that segwit did everything, including allow improved scalability and what confused me was that it was presented in a way that led me (and many others) to believe that segwit reduced the permanent storage needed.
now this is clarified that segwit does not reduce the space needed and that segwit softfork will force any node that wants to be able to validate setwit tx to also upgrade to segwit, then I think the rest is about implementation details.
And maybe someone can clarify the text on the bitcoincore.org site that presents segwit as curing cancer and world hunger?
James