A file that changed without changing size or timestamp is never transferred

A file that changed without changing size or timestamp is never transferred

reads as

`rsync -a src/ dst/` exits zero, and dst/app.conf exists with the same size and the same modification time as the source. Conclusion drawn: the destination is a copy of the source.

actually

rsync(1): rsync finds files that need to be transferred using a 'quick check' algorithm (by default) that looks for files that have changed in size or in last-modified time. A file edited in place to the same length, restored from an archive that preserved timestamps, or written by a generator that copies mtime from its input matches on both counts and is skipped. The old content remains, and the transfer is reported as complete because nothing needed transferring.

blind because

The two numbers the tool decides on are the two numbers used to verify it. Size and mtime agree, which is a true statement about the metadata and a false one about the contents.

the check

Compare contents rather than metadata: `rsync -ain --checksum src/ dst/` lists exactly what a content comparison would move. Observed on rsync 3.2.7 with src/app.conf holding VERSION=2 and dst/app.conf holding VERSION=1, both 10 bytes with mtime forced to 2026-01-01: `rsync -av src/ dst/` exited 0, reported `sent 72 bytes`, listed no files, and left the destination at VERSION=1. The same pair under `--checksum` transferred and the destination became VERSION=2. `cp -u` copied nothing for the same reason.

cost of missing

A deploy or backup succeeds and changes nothing, and repeating it reproduces the same success, so the obvious response to the symptom confirms the wrong hypothesis.

mitigation

Use `--checksum` for any sync where the content is authoritative and the timestamps are not, accepting the read cost; or ensure the writer touches mtime whenever it rewrites.

generalises to

Every change detector keyed on a proxy for content: mtime-based build systems, ETags derived from metadata, cache keys built from a version string that was not bumped.

source

man7.org