Blockchain Postgres Import

Introduction

This is Go code loosely aimed at importing the blockchain into Postgres as fast as possible.

This is work-in-progress and is subject to a lot of changes. Postgres import was our first goal and as part of it much blockchain-specific stuff had to be implemented which has nothing to do with Postgres, therefore it is likely that this project will be reorganized into more separate packages.

The goal here is to facilitate blockchain development using Go and PostgreSQL. There is also a sister project pg_blockchain which is a Postgres C language extension aimed at doing blockchain stuff inside the server. If this sounds interesting, please help us out by trying this out, we would very much appreciate any feedback, be it criticism, bug fixes or ideas.

We are focusing on Bitcoin initially, though this code should be generally applicable to any Bitcoin copycat out there as well.

Quick Overview

The source of the data is the Bitcoin Core store. You need a Core instance to download the entire blockchain, then the cmd/import tool will be able to read the data directly (not via RPC) by accessing the LevelDb and the blocks files as well as the UTXO set. (The Core program cannot run while this happens).

You should be able to build go build cmd/import/import.go then run it with (Core should not be running):

# Warning - this may take 12 hours
./import \
     -connstr "host=192.168.1.223 dbname=blocks sslmode=disable" \
     -blocks /whataver/blocks

This will read all blocks and upload them to Postgres. The block descriptors are first read from leveldb block index, which contains file names and offsets to actual block data. Using the block index lets us read blocks in order which is essential for the correct setting of tx_id in outputs. For every output we also query the UTXO set so that we can set the spent column correctly.

The output should look approximately like this:

2017/12/19 18:24:42 Setting open files rlimit of 256 to 1024.
2017/12/19 18:24:42 Reading block headers from LevelDb (/Volumes/SSD/Private/Bitcoin/blocks/index)...
2017/12/19 18:24:46 Ignoring orphan block 00000000000000000b5f97a18352ec38f9b5b20603c8fcd8f25ec9ee1ae0cf93
2017/12/19 18:24:46 Ignoring orphan block 0000000000000000014c463a40271798691071aba2c0d680403031bfdbcf0da3
2017/12/19 18:24:46 Ignoring orphan block 00000000000000000b945410c95718f36319521f6c22822a0032a813955a8198
2017/12/19 18:24:46 Ignoring orphan block 0000000000000000062459292110b290ae1312db9a3ca668b0c187181f6f27f3
2017/12/19 18:24:46 Ignoring orphan block 0000000000000000071412f6831a0f03b5abfb7490e8a4dfa13c82d6805096f1
2017/12/19 18:24:46 Ignoring orphan block 00000000000001e7f5686eedcf6fe2d27c7892b17c8b032e9b5c9241a9d8de32
2017/12/19 18:24:46 Read 499872 block headers.
2017/12/19 18:24:53 Height: 36276 Txs: 36525 Time: 2010-01-25 07:02:08 -0500 EST Tx/s: 7291.832654
2017/12/19 18:24:58 Height: 66780 Txs: 77685 Time: 2010-07-13 15:08:39 -0400 EDT Tx/s: 7761.014587
2017/12/19 18:25:03 Height: 86063 Txs: 130220 Time: 2010-10-18 12:37:57 -0400 EDT Tx/s: 8675.402340
2017/12/19 18:25:08 Height: 97697 Txs: 208377 Time: 2010-12-15 10:19:17 -0500 EST Tx/s: 10413.251706

  ... snip ...

2017/12/20 04:07:02 Height: 499853 Txs: 283295062 Time: 2017-12-17 17:10:19 -0500 EST Tx/s: 8108.974619
2017/12/20 04:07:06 Closed db channels, waiting for workers to finish...
2017/12/20 04:07:06 Block writer channel closed, leaving.
2017/12/20 04:07:06 TxOut writer channel closed, leaving.
2017/12/20 04:07:06 Tx writer channel closed, leaving.
2017/12/20 04:07:06 TxIn writer channel closed, leaving.
2017/12/20 04:07:06 Workers finished.
2017/12/20 04:07:06 Txid cache hits: 704485398 (100.00%) misses: 0 collisions: 0 dupes: 2 evictions: 253053982
2017/12/20 04:07:06 Creating indexes part 1 (if needed), please be patient, this may take a long time...
2017/12/20 04:07:06   - blocks primary key...
2017/12/20 04:07:07   - blocks prevhash index...
2017/12/20 04:07:08   - blocks hash index...
2017/12/20 04:07:09   - blocks height index...
2017/12/20 04:07:10   - txs primary key...
2017/12/20 04:22:19   - txs txid (hash) index...
2017/12/20 04:41:21   - block_txs block_id, n primary key...
2017/12/20 04:51:23   - block_txs tx_id index...
2017/12/20 04:58:07 NOT fixing missing prevout_tx_id entries because there were 0 cache misses.
2017/12/20 04:58:07 Creating indexes part 2 (if needed), please be patient, this may take a long time...
2017/12/20 04:58:07   - txins (prevout_tx_id, prevout_tx_n) index...
2017/12/20 06:32:35   - txins primary key...
2017/12/20 07:04:19   - txouts primary key...
2017/12/20 07:43:47 Creating constraints (if needed), please be patient, this may take a long time...
2017/12/20 07:43:47   - block_txs block_id foreign key...
2017/12/20 07:45:23   - block_txs tx_id foreign key...
2017/12/20 07:52:04   - txins tx_id foreign key...
2017/12/20 09:00:14   - txouts tx_id foreign key...
2017/12/20 10:20:47 Creating txins triggers.
2017/12/20 10:20:47 Marking orphan blocks...
2017/12/20 10:21:41 Indexes and constraints created.
2017/12/20 10:21:41 All done.

There are two phases to this process, the first is just streaming the data into Postgres, the second is building indexes, constraints and otherwise tying up loose ends.

The -cache-size parameter is the cache of txid (the SHA256) to the database tx_id, which import can set on the fly. This cache is also used to identify duplicate transactions. Having a cache of 30M (default) entries achieves 100% hit rate. The missing ids will be corrected later, but having as much as possible set from the beginning will reduce the time it takes to correct them later. A 30M entry cache will result in the import process taking up ~3GB of RAM.

After the initial import, the tool can "catch up" by importing new blocks not yet in the database. The catch up is many times slower than the initial import because it does not have the luxury of not having indexes and constraints.

PostgreSQL Tuning

• Using SSD's will make this process go much faster. Remember to set random_page_cost to 1 or less, depending on how fast your disk really is. The blockchain will occupy more than 250GB on disk and this will grow as time goes on.

• Turning off synchronous_commit and setting commit_delay to 100000 would make the import faster. Turning fsync off entirely might make it faster even still (heed the documentation warnings).

• shared_buffers should not be set high, PostgreSQL does better relying on the OS disk buffers cache. Shared buffers are faster than OS cache for hits, but more expensive on misses, thus the PG docs advise not relying on it unless the whole working set fits in PG shared buffers. Of course if your PG server has 512GB of RAM, then this advice does not apply.

• Setting maintenance_work_mem high should help with speeding up the index building. Note that it can be temporarily set right in the connection string (-connstr "host=... maintenance_work_mem=2GB").

• Setting wal_writer_delay to the max value of 10000 and increasing wal_buffers and wal_writer_flush_after should speed up the initial import in theory.

ZFS

Using a filesystem which supports snapshots is very useful for development of this thing because it provides the ability to quickly jump to a snapshot.

ZFS (at least used on a single disk) seems slower than ext4, but still well worth it. The settings we ended up with are:

zfs set compression=lz4 tank/blocks
zfs set atime=off tank/blocks
zfs set primarycache=all tank/blocks # yes, all, not metadata
zfs set recordsize=16k tank/blocks
zfs set logbias=throughput tank/blocks
echo 1 > /sys/module/zfs/parameters/zfs_txg_timeout

If you use ZFS, then in the Postgres config it is advisable to turn full_page_writes off.

Internals of the Data Stream

The initial data stream is done via COPY, with a separate goroutine streaming to its table. We read blocks in order, iterate over the transactions therein, the transactions are split into inputs, outpus, etc, and each of those records is sent over a channel to the goroutine responsible for that table. This approach is very performant.

On catch up the process is slightly more complicated because we need to ensure that referential integrity is maintained. Each block should be followed by a commit, all outputs in a block must be commited before inputs.