{"record":{"id":"ca913ff707da6285","repo":"juicedata/juicefs","slug":"failed-to-write-segment-length-s-w","errorCode":null,"errorMessage":"failed to write segment length %s: %w","messagePattern":"failed to write segment length (.+?): %w","errorType":"exception","errorClass":null,"httpStatus":null,"severity":"error","filePath":"pkg/meta/backup.go","lineNumber":339,"sourceCode":"\t\treturn 0\n\t}\n}\n\nfunc (s *BakSegment) Marshal(w io.Writer) (int, error) {\n\tif s == nil || s.val == nil {\n\t\treturn 0, fmt.Errorf(\"segment %s is nil\", s)\n\t}\n\n\tif err := binary.Write(w, binary.BigEndian, s.typ); err != nil {\n\t\treturn 0, fmt.Errorf(\"failed to write segment type %s : %w\", s, err)\n\t}\n\tdata, err := proto.Marshal(s.val)\n\tif err != nil {\n\t\treturn 0, fmt.Errorf(\"failed to marshal segment message %s : %w\", s, err)\n\t}\n\ts.len = uint64(len(data))\n\tif err := binary.Write(w, binary.BigEndian, s.len); err != nil {\n\t\treturn 0, fmt.Errorf(\"failed to write segment length %s: %w\", s, err)\n\t}\n\n\tif n, err := w.Write(data); err != nil || n != len(data) {\n\t\treturn 0, fmt.Errorf(\"failed to write segment data %s: err %w, write len %d, expect len %d\", s, err, n, len(data))\n\t}\n\n\treturn binary.Size(s.typ) + binary.Size(s.len) + len(data), nil\n}\n\nfunc (s *BakSegment) Unmarshal(r io.Reader) error {\n\tif err := binary.Read(r, binary.BigEndian, &s.typ); err != nil {\n\t\treturn fmt.Errorf(\"failed to read segment type: %v\", err)\n\t}\n\n\tif s.typ == BakEOS {\n\t\treturn errBakEOF\n\t}\n","sourceCodeStart":321,"sourceCodeEnd":357,"githubUrl":"https://github.com/juicedata/juicefs/blob/c9a67b23e8e08ec23ec331aa6f1675e2319e921c/pkg/meta/backup.go#L321-L357","documentation":"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.","triggerScenarios":"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.","commonSituations":"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.","solutions":["Check disk space/quota on the volume holding the backup output file and free space or retarget the backup","Verify the output file is writable and the descriptor hasn't been closed (check file permissions and open lifecycle)","Inspect the wrapped error (%w) in the message to identify the concrete I/O failure and address it","Retry the backup operation after fixing the storage issue"],"exampleFix":"// before\nf, err := os.Create(backupPath) // err ignored or file already closed elsewhere\n// after\nf, err := os.Create(backupPath)\nif err != nil { return err }\ndefer f.Close()\n// ensure enough free space: check via syscall.Statfs before writing","handlingStrategy":"try-catch","validationCode":"var st syscall.Statfs_t\nif err := syscall.Statfs(dir, &st); err == nil {\n    free := st.Bavail * uint64(st.Bsize)\n    if free < requiredBytes { return fmt.Errorf(\"insufficient space: %d < %d\", free, requiredBytes) }\n}","typeGuard":null,"tryCatchPattern":"n, err := seg.Marshal(w)\nif err != nil {\n    var pe *fs.PathError\n    if errors.As(err, &pe) { log.Printf(\"backup write failed on %s: %v\", pe.Path, pe.Err) }\n    return fmt.Errorf(\"backup footer write failed: %w\", err)\n}","preventionTips":["Check free disk space before running large metadata backups","Keep backup files on reliable storage and verify writability beforehand","Always inspect the wrapped %w cause to distinguish disk-full from closed-handle errors"],"tags":["io","write-failed","backup","protobuf"],"backgroundTag":"file-write-failed","analyzedSha":"c9a67b23e8e08ec23ec331aa6f1675e2319e921c","analyzedAt":"2026-09-06T17:55:48.476Z","contentChangedAt":"2026-09-06T17:55:48.476Z","schemaVersion":2},"datasetVersion":"2026-09-14T05:17:10.506Z"}