juicedata/juicefs · error

failed to write segment type %s : %w

Error message

failed to write segment type %s : %w

What it means

BakSegment.Marshal writes the 4-byte big-endian segment type as the first field of each segment record. This error wraps the underlying write failure from binary.Write into the segment's writer, meaning the destination writer (typically the buffered writer over the backup file) rejected the write.

Source

Thrown at pkg/meta/backup.go:331

			return uint64(len(b.Dirstats))
		case segTypeQuota:
			return uint64(len(b.Quotas) + len(b.UserQuotas) + len(b.GroupQuotas))
		case segTypeParent:
			return uint64(len(b.Parents))
		case segTypeChangeLog:
			return uint64(len(b.Changelogs))
		}
		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 {

View on GitHub (pinned to c9a67b23e8)

Solutions

1. Inspect the wrapped %w error for the root cause (ENOSPC, EPIPE, EACCES) and fix storage: free space, restore permissions, or reconnect the target.
2. Re-run the dump after fixing; the backup file is likely truncated/incomplete and must be rewritten from scratch.
3. If piping, verify the consumer process stayed alive and add pipefail so a dead consumer aborts the dump rather than silently truncating.
4. Write to a local staging file with sufficient free space, then copy to final destination, to avoid mid-stream failures on flaky mounts.

Example fix

// before: piping dump straight to a remote sink
juicefs dump meta://... | ssh host 'cat > meta.backup'

// after: stage locally, verify, then transfer
juicefs dump meta://... /local/meta.backup
if ! verifyFooter /local/meta.backup; then echo "dump failed"; exit 1; fi
scp /local/meta.backup host:

st, err := out.Stat()
if err == nil && st.Size()+estimateBytes > st.Sys().(*syscall.Stat_t).Blocks*512 { /* disk nearly full */ }
if freeDiskSpace(targetDir) < estimatedBackupSize { return errors.New("insufficient space for backup") }

func writerAlive(w io.Writer) bool {
    c, ok := w.(interface{ Check() error })
    return !ok || c.Check() == nil
}

if _, err := seg.Marshal(bw); err != nil {
    if isEPIPE(err) || isENOSPC(err) {
        os.Remove(backupPath) // discard incomplete backup
        return fmt.Errorf("backup write failed, target unusable: %w", err)
    }
    return err
}

When it happens

Trigger: The target file's write fails mid-backup: disk full, device error, file closed early, broken pipe on a piped/network writer, permissions revoked, or quota exceeded while `juicefs dump` streams segments.

Common situations: Dumping a large metadata tree to a full partition; writing to an NFS/S3-fuse mount that dropped; piping dump output (e.g. `juicefs dump ... | ssh`) where the remote end dies; running out of disk quota mid-write.

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 segment length %s: %w
• failed to write segment data %s: err %w, write len %d, expec
• write
• failed to write EOS: err %w, write len %d, expect len 4
• failed to write footer data: err %w, write len %d, expect le

AI-assisted analysis of juicedata/juicefs@c9a67b23e8 (2026-09-06). Data as JSON: /api/errors/fecbcf7511293c31. Report an issue: GitHub.