borgbackup/borg

borg2: make fastcdc chunker the new default?

Closed

#9,957 opened on Jul 27, 2026

View on GitHubContributor guide
 (1 comment) (0 reactions) (0 assignees)Python (732 forks)batch import
c: chunkerhelp wanted

Repository metrics

Stars
 (10,086 stars)
PR merge metrics
 (Avg merge 5d 12h) (91 merged PRs in 30d)

Description

It seems to be ~1.3x faster than buzhash64 (and buzhash), so for it could be the default.

As this is very new code, guess this would need some practical testing, so introduce the change while in beta, so it is practically well-tested when borg2 is released for production.

Of course we also have automated unit tests for fastcdc.

Contributor guide

Research direction
Investigate how the chunker is selected in borg2, locate the default chunker setting, and understand the code paths for fastcdc vs buzhash. Run the existing unit tests for fastcdc and benchmark the two chunkers to verify the performance claim.
Tech stack
python
Domain
backend
Issue type
Performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitPython
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65