grafana/k6 · error

failed to read the content of file %s: %w

Error message

failed to read the content of file %s: %w

What it means

The experimental fs module's cache opened the underlying file successfully but io.ReadAll failed while reading its content (internal/js/modules/k6/experimental/fs/cache.go:104). This is an I/O-level read failure after a successful open - the file exists and opened, but its bytes could not be fully read. Distinct from a missing file, which fails earlier at the Exists check with a NotFoundError.

Source

Thrown at internal/js/modules/k6/experimental/fs/cache.go:104

		return data, nil
	}

	// TODO: re-evaluate opening from the FS this once #1079 is resolved.
	f, err := fromFs.Open(filename)
	if err != nil {
		return nil, err
	}
	defer func() {
		cerr := f.Close()
		if cerr != nil {
			err = fmt.Errorf("failed to close file %s: %w", filename, cerr)
		}
	}()

	data, err = io.ReadAll(f)
	if err != nil {
		return nil, fmt.Errorf("failed to read the content of file %s: %w", filename, err)
	}

	fr.openedFiles.Store(filename, data)

	return data, nil
}

View on GitHub (pinned to 93accf6570)

Solutions

1. Ensure data files are not modified while k6 reads them (use write-to-temp-then-rename on the producer side)
2. Check file permissions and disk/volume health
3. Copy the file next to the script and re-run to rule out remote-storage issues

try {
  const file = open('./data.bin');
} catch (e) {
  if (/failed to read the content of file/.test(e.message)) {
    throw new Error(`data file unreadable: ${e.message}; verify it is not being rewritten during startup`);
  }
  throw e;
}

When it happens

Trigger: fs.open(path) where the file is truncated or replaced between the existence check and the read, or where the underlying afero/fsext layer returns a read error (permissions changed mid-run, disk errors, flaky remote volume).

Common situations: External processes rewriting data files while the test starts; containers with unreliable mounted volumes; files on network storage that stat fine but read intermittently.

Related errors

• failed to close file %s: %w
• open() failed, unable to verify if %q exists; reason: %w
• open() failed, unable to verify if %q is a directory; reason
• persisting screenshot: %w
• the 'header' option cannot be enabled when 'fromLine' is set

AI-assisted analysis of grafana/k6@93accf6570 (2026-08-15). Data as JSON: /api/errors/05f2b11d7dee1ff0. Report an issue: GitHub.