juicedata/juicefs · error

failed to write segment length %s: %w

Error message

failed to write segment length %s: %w

What it means

BakSegment.Marshal serializes a backup footer segment: it protobuf-marshals the segment's message, records its length in s.len, then writes the 8-byte big-endian length prefix to the output writer with binary.Write. This error is thrown when that binary.Write of the length prefix fails, wrapping the underlying writer error. It means the destination stream (file, buffer, network conn) refused or failed the write, not that the data was malformed.

Source

Thrown at pkg/meta/backup.go:339

		return 0
	}
}

func (s *BakSegment) Marshal(w io.Writer) (int, error) {
	if s == nil || s.val == nil {
		return 0, fmt.Errorf("segment %s is nil", s)
	}

	if err := binary.Write(w, binary.BigEndian, s.typ); err != nil {
		return 0, fmt.Errorf("failed to write segment type %s : %w", s, err)
	}
	data, err := proto.Marshal(s.val)
	if err != nil {
		return 0, fmt.Errorf("failed to marshal segment message %s : %w", s, err)
	}
	s.len = uint64(len(data))
	if err := binary.Write(w, binary.BigEndian, s.len); err != nil {
		return 0, fmt.Errorf("failed to write segment length %s: %w", s, err)
	}

	if n, err := w.Write(data); err != nil || n != len(data) {
		return 0, fmt.Errorf("failed to write segment data %s: err %w, write len %d, expect len %d", s, err, n, len(data))
	}

	return binary.Size(s.typ) + binary.Size(s.len) + len(data), nil
}

func (s *BakSegment) Unmarshal(r io.Reader) error {
	if err := binary.Read(r, binary.BigEndian, &s.typ); err != nil {
		return fmt.Errorf("failed to read segment type: %v", err)
	}

	if s.typ == BakEOS {
		return errBakEOF
	}

View on GitHub (pinned to c9a67b23e8)

Solutions

1. Check disk space/quota on the volume holding the backup output file and free space or retarget the backup
2. Verify the output file is writable and the descriptor hasn't been closed (check file permissions and open lifecycle)
3. Inspect the wrapped error (%w) in the message to identify the concrete I/O failure and address it
4. Retry the backup operation after fixing the storage issue

Example fix

// before
f, err := os.Create(backupPath) // err ignored or file already closed elsewhere
// after
f, err := os.Create(backupPath)
if err != nil { return err }
defer f.Close()
// ensure enough free space: check via syscall.Statfs before writing

var st syscall.Statfs_t
if err := syscall.Statfs(dir, &st); err == nil {
    free := st.Bavail * uint64(st.Bsize)
    if free < requiredBytes { return fmt.Errorf("insufficient space: %d < %d", free, requiredBytes) }
}

n, err := seg.Marshal(w)
if err != nil {
    var pe *fs.PathError
    if errors.As(err, &pe) { log.Printf("backup write failed on %s: %v", pe.Path, pe.Err) }
    return fmt.Errorf("backup footer write failed: %w", err)
}

When it happens

Trigger: Calling writeFooter (which invokes BakSegment.Marshal) while writing the backup footer to a writer that fails: disk full, closed file descriptor, I/O error on the underlying file, or a broken pipe if writing to a pipe/socket.

Common situations: Disk quota or full filesystem when writing a JuiceFS metadata backup file; backup target file closed or permission revoked mid-write; writing to a network stream that dropped.

Understand the failure class

Background: "failed to write file", "Could not save figure", "Error saving remote file" — file write failed: causes and fixes across languages and libraries — this error's family across 38 libraries.

Related errors

• failed to read footer: err %w, read len %d, expect len %d
• failed to write segment type %s : %w
• failed to write segment data %s: err %w, write len %d, expec
• write
• unknown message type %d

AI-assisted analysis of juicedata/juicefs@c9a67b23e8 (2026-09-06). Data as JSON: /api/errors/ca913ff707da6285. Report an issue: GitHub.