Function updateFile

core/snapshots/benchsuite/benchmark_test.go:285–316

updateFile modifies a few bytes in the middle in order to demonstrate the difference in performance for block-based snapshotters (like devicemapper) against file-based snapshotters (like overlay, which need to perform a copy-up of the full file any time a single bit is modified).

(name string)

// for block-based snapshotters (like devicemapper) against file-based snapshotters (like overlay, which need to
// perform a copy-up of the full file any time a single bit is modified).
func updateFile(name string) applierFn {
	return func(root string) error {
		path := filepath.Join(root, name)
		file, err := os.OpenFile(path, os.O_WRONLY, 0600)
		if err != nil {
			return fmt.Errorf("failed to open %q: %w", path, err)
		}

		info, err := file.Stat()
		if err != nil {
			file.Close()
			return err
		}

		var (
			offset = info.Size() / 2
			buf    = make([]byte, 4)
		)

		if _, err := rand.Read(buf); err != nil {
			file.Close()
			return err
		}

		if _, err := file.WriteAt(buf, offset); err != nil {
			file.Close()
			return fmt.Errorf("failed to write %q at offset %d: %w", path, offset, err)
		}

		return file.Close()
	}
}