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From: Bram Cohen <bram@bittorrent.com>
Date: Sun, 2 Apr 2017 20:13:52 -0700
Message-ID: <CA+KqGkq4BNUpwsxYnc4d-RSpq7-og4Xf2zwtUC-AVBjF4_u7fg@mail.gmail.com>
To: praxeology_guy <praxeology_guy@protonmail.com>, 
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Content-Type: multipart/alternative; boundary=001a113f8a74845f85054c3a8c3d
Subject: Re: [bitcoin-dev] Guessing the spentness status of the pruned
	relatives
Precedence: list

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On Sun, Apr 2, 2017 at 1:43 PM, praxeology_guy via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> TL;DR: using spentness bits scales linearly... vs swapping digest leafs
> with empties can scale with logorithmically increasing storage
> requirements.  So if you are using a 32 byte hash and spentness bits, you
> are pretty much limited to only being able to prune 8 to 12 layers.  This
> corresponds to an MMR proof length of 512 to 768 bytes.
>

Yes the point of the txo bitfield is that the constant factors are so good
that it's entirely under control. Making the memory commitments smaller
requires that the proofs be kept up to date and increases CPU requirements
and proof size. It would be entirely reasonable to make an MMRs of the
bitfield or the insertion index data structure but they aren't needed
immediately if ever. For the insertion ordering structure it's reasonable
to require full nodes to cache the top bunch of layers to make the proofs
smaller, but a very expedient approximation of that is to make them simply
remember a root per block for all the insertions contained therein, and
have full nodes remember all of those.

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Content-Transfer-Encoding: quoted-printable

<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote">On S=
un, Apr 2, 2017 at 1:43 PM, praxeology_guy via bitcoin-dev <span dir=3D"ltr=
">&lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_b=
lank">bitcoin-dev@lists.linuxfoundation.org</a>&gt;</span> wrote:<br><block=
quote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc=
 solid;padding-left:1ex"><div>TL;DR: using spentness bits scales linearly..=
. vs swapping digest leafs with empties can scale with logorithmically incr=
easing storage requirements.=C2=A0 So if you are using a 32 byte hash and s=
pentness bits, you are pretty much limited to only being able to prune 8 to=
 12 layers.=C2=A0 This corresponds to an MMR proof length of 512 to 768 byt=
es.<br></div></blockquote><div><br></div><div>Yes the point of the txo bitf=
ield is that the constant factors are so good that it&#39;s entirely under =
control. Making the memory commitments smaller requires that the proofs be =
kept up to date and increases CPU requirements and proof size. It would be =
entirely reasonable to make an MMRs of the bitfield or the insertion index =
data structure but they aren&#39;t needed immediately if ever. For the inse=
rtion ordering structure it&#39;s reasonable to require full nodes to cache=
 the top bunch of layers to make the proofs smaller, but a very expedient a=
pproximation of that is to make them simply remember a root per block for a=
ll the insertions contained therein, and have full nodes remember all of th=
ose.</div></div></div></div>

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