73 lines
4.4 KiB
Markdown
73 lines
4.4 KiB
Markdown
---
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title: Block chain structure on disk.
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...
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The question is: One enormous SQLite file, or actually store the chain as a collection of files?
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In the minimum viable product, the blockchain will be quite small, and it
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will be workable to put it one big SQLite file. The trouble with one enormous
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SQLite file is that when it gets big enough, we face a high and steadily
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increasing risk of one sector on the enormous disk going bad, corrupting the
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entire database. SQLite does not handle the loss of a single sector gracefully.
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We will eventually need our own database structure designed around
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Merkle-patricia trees, append only data structures, and accommodating a near
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certainty of sectors and entire disks continually going bad. When one hundred
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disks have to be added every year, entire disks will be failing every day or
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so, and sectors will be failing every second.
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Eventually, a typical peer will have several big racks of disks. When we
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replace the world monetary system, twenty servers each with twenty disks, two
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hundred thousand transaction inputs and outputs a second, (for each
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transaction minimally involves one input and two outputs, a change output and
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a payment output, and usually a lot more. Each signature is sixty four bytes.
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Each input and output is at least forty bytes. So, say, on average two inputs
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and two outputs per payment – say, perhaps 288 bytes per payment, and we will
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want to do one hundred thousand payments per second. So, about nine hundred
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terabytes a year. With 2020 disk technology, that is about seventy five twelve
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terabyte hard drives per year, costing about one hundred and fifty hard drives
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per year costing fifty five thousand dollars per year, to store all the
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transactions of the world forever.
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If we are constructing one block per five minutes, each block is about ten
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gigabytes. Sqlite3 cannot possibly handle that – the blocks are going to have
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to be dispersed over many drives and many physical computers. We are going to
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have to go to our own custom low level format, in which a block is distributed
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over many drives and many servers, the upper part of the block Merkle-patricia
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tree duplicated on every shard, but the the lower branches of the tree each in
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a separate shard. Instead of a file structure with many files on one enormous
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disk, we have one enormous data structure on servers, each server with many
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disks.
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Optimal solution is to store recently accessed data in one big SQLite file,
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while also storing the data in a large collection of blocks, once it has become
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subject to wide consensus. Older blocks, fully incorporated in the current
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consensus, get written to disk in our own custom Merkle-patricia tree format,
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with append only Merkle-patricia tree node locations, [a sequential append only
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collection of binary trees in postfix tree format](
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merkle_patricia-dac.html#a-sequential-append-only-collection-of-postfix-binary-trees).
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Each file, incorporating a
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range of blocks, has its location on disk, time, size, and the roots of its
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Merkle-patricia trees recorded in the SQL database. On program launch, the
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size, touch time, and root has of newest block in the file are checked. If
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there is a discrepancy, we do a full check of the Merkle-patricia tree, editing
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it as necessary to an incomplete Merkle-patricia tree, download missing data
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from peers, and rebuild the blocks, thus winding up with a newer touch dates.
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Our per peer configuration file tells us where to find the block files, and if
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they are not stored where expected, we rebuild. If stored where expected, but
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touch dates unavailable or incorrect (perhaps because this is the first time the
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program launched) then the entire system of Merkle-patricia trees is validated,
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making sure the data on disk is consistent.
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How do we tell the one true blockchain, from some other evil blockchain?
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Well, the running definition is consensus, that you can interact with other
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peers because they agree on the running root hash. So you downloaded this
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software from somewhere, and when you downloaded it, you got the means to
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contact a bunch of peers, whom we suppose agree, and each have evidence that
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other peers agree. And, having downloaded what they agree on, you then treat
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it as gospel and as more authoritative that what others say, so long a touch
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dates, file sizes, locations, and the hash of the most recent block in the file
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are consistent, and the internal contents of each file are consistent with root
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of the most recent tree.
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