juicedata/juicefs · error

failed to write EOS: err %w, write len %d, expect len 4

Error message

failed to write EOS: err %w, write len %d, expect len 4

What it means

writeEOS appends the 4-byte big-endian End-Of-Sections marker to the backup stream. The error fires when the io.Writer returns an error or a short write (n != 4), meaning the destination stream failed mid-write.

Source

Thrown at pkg/meta/backup.go:159

func (f *BakFormat) ReadSegment(r io.Reader) (*BakSegment, error) {
	seg := &BakSegment{}
	if err := seg.Unmarshal(r); err != nil {
		return nil, err
	}
	return seg, nil
}

func (f *BakFormat) writeFooter(w io.Writer) error {
	if err := f.writeEOS(w); err != nil {
		return err
	}
	return f.Footer.Marshal(w)
}

func (f *BakFormat) writeEOS(w io.Writer) error {
	if n, err := w.Write(binary.BigEndian.AppendUint32(nil, BakEOS)); err != nil && n != 4 {
		return fmt.Errorf("failed to write EOS: err %w, write len %d, expect len 4", err, n)
	}
	return nil
}

func (f *BakFormat) ReadFooter(r io.ReadSeeker) (*BakFooter, error) { // nolint:unused
	footer := &BakFooter{}
	if err := footer.Unmarshal(r); err != nil {
		return nil, err
	}
	if footer.Msg.Magic != BakMagic {
		return nil, fmt.Errorf("invalid magic number %d, expect %d", footer.Msg.Magic, BakMagic)
	}
	f.Footer = footer
	return footer, nil
}

type BakFooter struct {
	Msg *pb.Footer

View on GitHub (pinned to c9a67b23e8)

Solutions

1. Check free space on the output destination and retry the dump
2. If streaming, verify the connection stayed open for the whole dump (timeouts, proxies)
3. Retry the dump writing to a local file first, then upload the completed file

// pre-flight: ensure destination has space
st, err := os.Stat(destDir) // plus free-space check
if free < expectedBackupSize { return errors.New("insufficient space for backup") }

if err := f.writeFooter(w); err != nil {
    if strings.Contains(err.Error(), "failed to write EOS") || strings.Contains(err.Error(), "short") {
        return fmt.Errorf("destination stream failed mid-dump, retry: %w", err)
    }
    return err
}

When it happens

Trigger: Calling writeFooter → writeEOS when the underlying writer fails: disk full on the backup output file, closed connection to a network stream (HTTP body, pipe), or the writer returning io.ErrShortWrite.

Common situations: Dumping metadata to a full disk or quota-limited volume; writing a backup over an interrupted network stream (S3 multipart, HTTP upload); piping to a process that exited early (broken pipe).

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 write footer data: err %w, write len %d, expect le
• failed to write footer length: err %w, write len %d, expect
• failed to write segment data %s: err %w, write len %d, expec
• failed to read footer length: err %w, read len %d, expect le
• failed to read footer: err %w, read len %d, expect len %d

AI-assisted analysis of juicedata/juicefs@c9a67b23e8 (2026-09-06). Data as JSON: /api/errors/a116739e11465069. Report an issue: GitHub.