Not entirely correct, no. Edge cases also matter. Segwit is described as
4MB because that is the largest possible combined block size that can be
constructed. BIP 102 + segwit would allow a maximum relay of 8MB. So you
have to be confident that an 8MB relay size would be acceptable, even if a
block full of actual transactions would be closer to 3.5MB.

On Fri, Dec 18, 2015 at 6:01 PM, sickpig--- via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Anthony,
>
>
> On Thu, Dec 17, 2015 at 6:55 PM, Anthony Towns via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> On Thu, Dec 17, 2015 at 04:51:19PM +0100, sickpig--- via bitcoin-dev
>> wrote:
>> > On Thu, Dec 17, 2015 at 2:09 PM, Jorge Timón wrote:
>> > > Unless I'm missing something, 2 mb x4 = 8mb, so bip102 + SW is already
>> > > equivalent to the 2-4-8 "compromise" proposal [...]
>> > isn't SegWit gain ~75%? hence 2mb x 1.75 = 3.5.
>>
>> Segwit as proposed gives a 75% *discount* to witness data with the
>> same limit, so at a 1MB limit, that might give you (eg) 2.05MB made up
>> of 650kB of base block data plus 1.4MB of witness data; where 650kB +
>> 1.4MB/4 = 1MB at the 1MB limit; or 4.1MB made up of 1.3MB of base plus
>> 2.8MB of witness, for 1.3MB+2.8MB/4 = 2MB at a 2MB limit.
>>
>> > 4x is theoric gain you get in case of 2-2 multisig txs.
>>
>> With segregated witness, 2-2 multisig transactions are made up of 94B
>> of base data, plus about 214B of witness data; discounting the witness
>> data by 75% gives 94+214/4=148 bytes. That compares to about 301B for
>> a 2-2 multisig transaction with P2SH rather than segwit, and 301/148
>> gives about a 2.03x gain, not a 4x gain. A 2.05x gain is what I assumed
>> to get the numbers above.
>>
>> You get further improvements with, eg, 3-of-3 multisig, but to get
>> the full, theoretical 4x gain you'd need a fairly degenerate looking
>> transaction.
>>
>> Pay to public key hash with segwit lets you move about half the
>> transaction data into the witness, giving about a 1.6x improvement by
>> my count (eg 1.6MB = 800kB of base data plus 800kB of witness data,
>> where 800kB+800kB/4=1MB), so I think a gain of between 1.6 and 2.0 is
>> a reasonable expectation to have for the proposed segwit scheme overall.
>>
>>
> many thanks for the explanation.
>
> so it should be fair to say that BIP 102 + SW would bring a gain between
> 2*1.6 and 2*2.
>
> Just for the sake of simplicity if we take the middle of the interval we
> could say
> that BIP102 + SW will bring us a max block (virtual) size equal to 1MB * 2
> * 1.8 = 3.6
>
> Is it right?
>
>
>
>
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