下面来看成员函数，versionset的细节在函数中讲解：

void VersionSet::AppendVersion(Version* v) { // Make "v" current assert(v->refs_ == 0); assert(v != current_); if (current_ != NULL) { current_->Unref(); } current_ = v; v->Ref(); // Append to linked list v->prev_ = dummy_versions_.prev_; v->next_ = &dummy_versions_; v->prev_->next_ = v; v->next_->prev_ = v; }

添加version，所有的version是用一个环状链表保存指针的，添加version也就是更新dummy_versions_的前后指针，并且设置V为当前version（current_）

Status VersionSet::LogAndApply(VersionEdit* edit, port::Mutex* mu) { if (edit->has_log_number_) { assert(edit->log_number_ >= log_number_); assert(edit->log_number_ < next_file_number_); } else { edit->SetLogNumber(log_number_); } if (!edit->has_prev_log_number_) { edit->SetPrevLogNumber(prev_log_number_); } edit->SetNextFile(next_file_number_); edit->SetLastSequence(last_sequence_); Version* v = new Version(this); { Builder builder(this, current_); builder.Apply(edit); builder.SaveTo(v); } Finalize(v); // Initialize new descriptor log file if necessary by creating // a temporary file that contains a snapshot of the current version. std::string new_manifest_file; Status s; if (descriptor_log_ == NULL) { // No reason to unlock *mu here since we only hit this path in the // first call to LogAndApply (when opening the database). assert(descriptor_file_ == NULL); new_manifest_file = DescriptorFileName(dbname_, manifest_file_number_); edit->SetNextFile(next_file_number_); s = env_->NewWritableFile(new_manifest_file, &descriptor_file_); if (s.ok()) { descriptor_log_ = new log::Writer(descriptor_file_); s = WriteSnapshot(descriptor_log_); } } // Unlock during expensive MANIFEST log write { mu->Unlock(); // Write new record to MANIFEST log if (s.ok()) { std::string record; edit->EncodeTo(&record); s = descriptor_log_->AddRecord(record); if (s.ok()) { s = descriptor_file_->Sync(); } if (!s.ok()) { Log(options_->info_log, "MANIFEST write: %s\n", s.ToString().c_str()); } } // If we just created a new descriptor file, install it by writing a // new CURRENT file that points to it. if (s.ok() && !new_manifest_file.empty()) { s = SetCurrentFile(env_, dbname_, manifest_file_number_); } mu->Lock(); } // Install the new version if (s.ok()) { AppendVersion(v); log_number_ = edit->log_number_; prev_log_number_ = edit->prev_log_number_; } else { delete v; if (!new_manifest_file.empty()) { delete descriptor_log_; delete descriptor_file_; descriptor_log_ = NULL; descriptor_file_ = NULL; env_->DeleteFile(new_manifest_file); } } return s; }

产生新版本并更新其中files_[]：
首先输入参数VersionEdit记录了leveldb每次更新的信息，包括新加入的sstable以及要删除的sstable，则只需利用VersionSet::Builder更新新产生的version对象，然后将其加入version的链表中，并指定为current_。
然后还要向manifest文件写入新version的信息（为了重启db后可以恢复推出前的状态，需要将db中的状态保存下来，这些信息就保存在manfest文件中）

void VersionSet::Finalize(Version* v)

对version中的每一级sstable做一个评估，选择score最高的level作为需要compact的level（compaction_level_），评估是根据每个level上文件的大小（level 你，n>0）和数量(level 0)