mirror of
https://github.com/borgbackup/borg.git
synced 2026-07-08 01:31:04 -04:00
f2b96e3d15
Add a new "fastcdc" content-defined chunker selectable via --chunker-params.
It uses the FastCDC Gear rolling hash (fp = (fp << 1) + Gear[byte]), which is
window-less and cheaper per byte than buzhash's cyclic-polynomial update, so it
chunks noticeably faster (see "borg benchmark cpu" output), while producing
the same chunk-size distribution and deduplication.
The Gear table is keyed: it is derived from the repo id key via CSPRNG (own
"fastcdc" domain), exactly like the buzhash64 table, so chunk cut points stay
unpredictable without the key (anti-fingerprinting). It implements the same
FastCDC techniques as buzhash64 (sub-minimum skipping, normalized chunking with
a required nc_level, min/max clamping); the mask uses the high bits of the hash
(Gear accumulates entropy there).
chunker-params: "fastcdc,chunk_min,chunk_max,chunk_mask,nc_level" - there is no
window field, because Gear is window-less. e.g. fastcdc,19,23,21,2
Also: borg benchmark cpu now measures the fastcdc chunker; tests in
borg.testsuite.chunkers (golden vector, size distribution, keyed gear table,
param parsing, slow fuzz); docs and changelog.
Benchmarks (scripts/chunker_bench.py, buzhash64 vs fastcdc, both nc_level=2,
incompressible data unless noted):
5 GiB, 2 MiB target (default params):
buzhash64: CV 0.294, 1011 MB/s
fastcdc: CV 0.295, 1313 MB/s (+30%)
64 MiB, 64 KiB target:
buzhash64: CV 0.374, shift-resilience 0.9928, 963 MB/s
fastcdc: CV 0.359, shift-resilience 0.9929, 1331 MB/s (+38%)
Re-backup of a 2.5 GiB file after scattered single-byte edits (dedup ratio,
0.5 = v2 fully deduplicated, lower is better):
64 edits: buzhash64 0.5237, fastcdc 0.5236
320 edits: buzhash64 0.6133, fastcdc 0.6161
borg benchmark cpu, 1 GB: fastcdc 3.80s, buzhash 4.36s, buzhash64 8.13s,
fixed 0.56s.
Chunk-size distribution, deduplication and shift-resilience match buzhash64
within noise; fastcdc is consistently faster.
Also: fix bug when computing the mask, one needs to use 1ULL instead of
1, so the shifting computation is done in a uint64, not in a 32bit int.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
|
||
|---|---|---|
| .. | ||
| make-testdata | ||
| shell_completions/fish | ||
| borg.exe.spec | ||
| build-borg-using-nuitka.sh | ||
| build-borg-using-pyinstaller.sh | ||
| chunker_bench.py | ||
| Dockerfile.linux-run | ||
| errorlist.py | ||
| fetch-binaries | ||
| glibc_check.py | ||
| linux-run | ||
| make.py | ||
| msys2-install-deps | ||
| sdist-sign | ||
| sign-binaries | ||
| upload-pypi | ||