This change is part of [[IBD] - Tracking PR for speeding up Initial Block Download](https://github.com/bitcoin/bitcoin/pull/32043) ### Summary We can serialize the blocks and undos to any `Stream` which implements the appropriate read/write methods. `AutoFile` is one of these, writing the results "directly" to disk (through the OS file cache). Batching these in memory first and reading/writing these to disk is measurably faster (likely because of fewer native fread calls or less locking, as [observed](https://github.com/bitcoin/bitcoin/pull/28226#issuecomment-1666842501) by Martinus in a similar change). ### Unlocking new optimization opportunities Buffered writes will also enable batched obfuscation calculations (implemented in https://github.com/bitcoin/bitcoin/pull/31144) - especially since currently we need to copy the write input's std::span to do the obfuscation on it, and batching enables doing the operations on the internal buffer directly. ### Measurements (micro benchmarks, full IBDs and reindexes) Microbenchmarks for `[Read|Write]BlockBench` show a ~**30%**/**168%** speedup with `macOS/Clang`, and ~**19%**/**24%** with `Linux/GCC` (the follow-up XOR batching improves these further):

macOS Sequoia - Clang 19.1.7

> Before: | ns/op | op/s | err% | total | benchmark |--------------------:|--------------------:|--------:|----------:|:---------- | 2,271,441.67 | 440.25 | 0.1% | 11.00 | `ReadBlockBench` | 5,149,564.31 | 194.19 | 0.8% | 10.95 | `WriteBlockBench` > After: | ns/op | op/s | err% | total | benchmark |--------------------:|--------------------:|--------:|----------:|:---------- | 1,738,683.04 | 575.15 | 0.2% | 11.04 | `ReadBlockBench` | 3,052,658.88 | 327.58 | 1.0% | 10.91 | `WriteBlockBench`

Ubuntu 24 - GNU 13.3.0

> Before: | ns/op | op/s | err% | ins/op | cyc/op | IPC | bra/op | miss% | total | benchmark |--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:---------- | 6,895,987.11 | 145.01 | 0.0% | 71,055,269.86 | 23,977,374.37 | 2.963 | 5,074,828.78 | 0.4% | 22.00 | `ReadBlockBench` | 5,152,973.58 | 194.06 | 2.2% | 19,350,886.41 | 8,784,539.75 | 2.203 | 3,079,335.21 | 0.4% | 23.18 | `WriteBlockBench` > After: | ns/op | op/s | err% | ins/op | cyc/op | IPC | bra/op | miss% | total | benchmark |--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:---------- | 5,771,882.71 | 173.25 | 0.0% | 65,741,889.82 | 20,453,232.33 | 3.214 | 3,971,321.75 | 0.3% | 22.01 | `ReadBlockBench` | 4,145,681.13 | 241.21 | 4.0% | 15,337,596.85 | 5,732,186.47 | 2.676 | 2,239,662.64 | 0.1% | 23.94 | `WriteBlockBench`

2 full IBD runs against master (compiled with GCC where the gains seem more modest) for **888888** blocks (seeded from real nodes) indicates a ~**7%** total speedup.

Details

```bash COMMITS="d2b72b13699cf460ffbcb1028bcf5f3b07d3b73a 652b4e3de5c5e09fb812abe265f4a8946fa96b54"; \ STOP_HEIGHT=888888; DBCACHE=1000; \ C_COMPILER=gcc; CXX_COMPILER=g++; \ BASE_DIR="/mnt/my_storage"; DATA_DIR="$BASE_DIR/BitcoinData"; LOG_DIR="$BASE_DIR/logs"; \ (for c in $COMMITS; do git fetch origin $c -q && git log -1 --pretty=format:'%h %s' $c || exit 1; done) && \ hyperfine \ --sort 'command' \ --runs 2 \ --export-json "$BASE_DIR/ibd-${COMMITS// /-}-$STOP_HEIGHT-$DBCACHE-$C_COMPILER.json" \ --parameter-list COMMIT ${COMMITS// /,} \ --prepare "killall bitcoind; rm -rf $DATA_DIR/*; git checkout {COMMIT}; git clean -fxd; git reset --hard; \ cmake -B build -DCMAKE_BUILD_TYPE=Release -DENABLE_WALLET=OFF -DCMAKE_C_COMPILER=$C_COMPILER -DCMAKE_CXX_COMPILER=$CXX_COMPILER && \ cmake --build build -j$(nproc) --target bitcoind && \ ./build/bin/bitcoind -datadir=$DATA_DIR -stopatheight=1 -printtoconsole=0; sleep 100" \ --cleanup "cp $DATA_DIR/debug.log $LOG_DIR/debug-{COMMIT}-$(date +%s).log" \ "COMPILER=$C_COMPILER COMMIT=${COMMIT:0:10} ./build/bin/bitcoind -datadir=$DATA_DIR -stopatheight=$STOP_HEIGHT -dbcache=$DBCACHE -blocksonly -printtoconsole=0" d2b72b1369 refactor: rename leftover WriteBlockBench 652b4e3de5 optimization: Bulk serialization writes in `WriteBlockUndo` and `WriteBlock` Benchmark 1: COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=888888 -dbcache=1000 -blocksonly -printtoconsole=0 (COMMIT = d2b72b13699cf460ffbcb1028bcf5f3b07d3b73a) Time (mean ± σ): 41528.104 s ± 354.003 s [User: 44324.407 s, System: 3074.829 s] Range (min … max): 41277.786 s … 41778.421 s 2 runs Benchmark 2: COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=888888 -dbcache=1000 -blocksonly -printtoconsole=0 (COMMIT = 652b4e3de5c5e09fb812abe265f4a8946fa96b54) Time (mean ± σ): 38771.457 s ± 441.941 s [User: 41930.651 s, System: 3222.664 s] Range (min … max): 38458.957 s … 39083.957 s 2 runs Relative speed comparison 1.07 ± 0.02 COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=888888 -dbcache=1000 -blocksonly -printtoconsole=0 (COMMIT = d2b72b13699cf460ffbcb1028bcf5f3b07d3b73a) 1.00 COMPILER=gcc ./build/bin/bitcoind -datadir=/mnt/my_storage/BitcoinData -stopatheight=888888 -dbcache=1000 -blocksonly -printtoconsole=0 (COMMIT = 652b4e3de5c5e09fb812abe265f4a8946fa96b54) ```

[IBD] batch block reads/writes during `AutoFile` serialization