leveldb - menifest文件格式

MANIFEST文件是Level DB的元信息文件，它主要包括下面一些信息：

void VersionEdit::EncodeTo(std::string* dst) const {
  if (has_comparator_) {  // 记录Comparator
    PutVarint32(dst, kComparator);
    PutLengthPrefixedSlice(dst, comparator_);
  }
  if (has_log_number_) {  // 记录Log Numer
    PutVarint32(dst, kLogNumber);
    PutVarint64(dst, log_number_);
  }
  if (has_prev_log_number_) {  // 记录Prev Log Number，现在已废弃，一般为0
    PutVarint32(dst, kPrevLogNumber);
    PutVarint64(dst, prev_log_number_);
  }
  if (has_next_file_number_) {  // 记录下一个文件序号
    PutVarint32(dst, kNextFileNumber);
    PutVarint64(dst, next_file_number_);
  }
  if (has_last_sequence_) {  // 记录最大的sequence num
    PutVarint32(dst, kLastSequence);
    PutVarint64(dst, last_sequence_);
  }

  // 记录每一级Level下次compaction的起始Key
  for (size_t i = 0; i < compact_pointers_.size(); i++) {
    PutVarint32(dst, kCompactPointer);
    PutVarint32(dst, compact_pointers_[i].first);  // level
    PutLengthPrefixedSlice(dst, compact_pointers_[i].second.Encode());
  }
  // 记录每一级需要删除的文件
  for (DeletedFileSet::const_iterator iter = deleted_files_.begin();
       iter != deleted_files_.end();
       ++iter) {
    PutVarint32(dst, kDeletedFile);
    PutVarint32(dst, iter->first);   // level
    PutVarint64(dst, iter->second);  // file number
  }
  // 记录每一级需要有效的sst以及其smallest与largest的key
  for (size_t i = 0; i < new_files_.size(); i++) {
    const FileMetaData& f = new_files_[i].second;
    PutVarint32(dst, kNewFile);
    PutVarint32(dst, new_files_[i].first);  // level
    PutVarint64(dst, f.number);
    PutVarint64(dst, f.file_size);
    PutLengthPrefixedSlice(dst, f.smallest.Encode());
    PutLengthPrefixedSlice(dst, f.largest.Encode());
  }
}