简介
在RocksDB中，每次写入它都会先写WAL,然后再写入MemTable,这次我们就来分析这两个逻辑具体是如何实现的. 首先需要明确的是在RocksDB中，WAL的写入是单线程顺序串行写入的，而MemTable则是可以并发多线程写入的。
而在RocksDB 5.5中引进了一个选项enable_pipelined_write，这个选项的目的就是将WAL和MemTable的写入pipeline化， 也就是说当一个线程写完毕WAL之后，此时在WAL的write队列中等待的其他的write则会开始继续写入WAL, 而当前线程将会继续 写入MemTable.此时就将不同的Writer的写入WAL和写入MemTable并发执行了.
实现
我们这里只来分析pipeline的实现，核心函数就是DBImpl::PipelinedWriteImpl.
- 每一个DB(DBImpl)都有一个write_thread_(class WriteThread).
- 每次调用Write的时候会先写入WAL, 此时新建一个WriteThread::Writer对象，并将这个对象加入到一个Group中(调用JoinBatchGroup)
WriteThread::Writer w(write_options, my_batch, callback, log_ref, disable_memtable); write_thread_.JoinBatchGroup(&w); - 然后我们来看JoinBatchGroup,这个函数主要是用来讲所有的写入WAL加入到一个Group中.这里可以看到当当前的Writer 对象是leader(比如第一个进入的对象)的时候将会直接返回，否则将会等待知道更新为对应的状态．
void WriteThread::JoinBatchGroup(Writer* w) { ................................... bool linked_as_leader = LinkOne(w, &newest_writer_); if (linked_as_leader) { SetState(w, STATE_GROUP_LEADER); } TEST_SYNC_POINT_CALLBACK("WriteThread::JoinBatchGroup:Wait", w); if (!linked_as_leader) { /** * Wait util: * 1) An existing leader pick us as the new leader when it finishes * 2) An existing leader pick us as its follewer and * 2.1) finishes the memtable writes on our behalf * 2.2) Or tell us to finish the memtable writes in pralallel * 3) (pipelined write) An existing leader pick us as its follower and * finish book-keeping and WAL write for us, enqueue us as pending * memtable writer, and * 3.1) we become memtable writer group leader, or * 3.2) an existing memtable writer group leader tell us to finish memtable * writes in parallel. */ AwaitState(w, STATE_GROUP_LEADER | STATE_MEMTABLE_WRITER_LEADER | STATE_PARALLEL_MEMTABLE_WRITER | STATE_COMPLETED, &jbg_ctx); TEST_SYNC_POINT_CALLBACK("WriteThread::JoinBatchGroup:DoneWaiting", w); } } - 然后我们来看LinkOne函数，这个函数主要用来讲当前的Writer对象加入到group中，这里可以看到由于 写入是并发的因此对应的newest_writer_(保存最新的写入对象)需要原子操作来更新.
bool WriteThread::LinkOne(Writer* w, std::atomic<Writer*>* newest_writer) { assert(newest_writer != nullptr); assert(w->state == STATE_INIT); Writer* writers = newest_writer->load(std::memory_order_relaxed); while (true) { w->link_older = writers; if (newest_writer->compare_exchange_weak(writers, w)) { return (writers == nullptr); } } } - 当从JoinBatchGroup返回之后，当当前的Writer对象为leader的话，则将会把此leader下的所有的write都 链接到一个WriteGroup中(调用EnterAsBatchGroupLeader函数), 并开始写入WAL,这里要注意非leader的write将会直接 进入memtable的写入，这是因为非leader的write都将会被当前它所从属的leader来打包(group)写入，后面我们会看到实现.
size_t WriteThread::EnterAsBatchGroupLeader(Writer* leader, WriteGroup* write_group) { assert(leader->link_older == nullptr); assert(leader->batch != nullptr); assert(write_group != nullptr); ................................................ Writer* newest_writer = newest_writer_.load(std::memory_order_acquire); // This is safe regardless of any db mutex status of the caller. Previous // calls to ExitAsGroupLeader either didn't call CreateMissingNewerLinks // (they emptied the list and then we added ourself as leader) or had to // explicitly wake us up (the list was non-empty when we added ourself, // so we have already received our MarkJoined). CreateMissingNewerLinks(newest_writer); // Tricky. Iteration start (leader) is exclusive and finish // (newest_writer) is inclusive. Iteration goes from old to new. Writer* w = leader; while (w != newest_writer) { w = w->link_newer; ......................................... w->write_group = write_group; size += batch_size; write_group->last_writer = w; write_group->size++; } .............................. } - 这里注意到遍历是通过link_newer进行的，之所以这样做是相当于在写入WAL之前，对于当前leader的Write 做一次snapshot(通过CreateMissingNewerLinks函数).
void WriteThread::CreateMissingNewerLinks(Writer* head) { while (true) { Writer* next = head->link_older; if (next == nullptr || next->link_newer != nullptr) { assert(next == nullptr || next->link_newer == head); break; } next->link_newer = head; head = next; } } - 上述操作进行完毕之后，进入写WAL操作,最终会把这个write_group打包成一个writeBatch(通过MergeBatch函数)进行写入.
if (w.ShouldWriteToWAL()) {
...............................
w.status = WriteToWAL(wal_write_group, log_writer, log_used,
need_log_sync, need_log_dir_sync, current_sequence);
}
- 当当前的leader将它自己与它的follow写入之后，此时它将需要写入memtable,那么此时之前还阻塞的Writer，分为两种情况 第一种是已经被当前的leader打包写入到WAL，这些writer(包括leader自己)需要将他们链接到memtable writer list.还有一种情况，那就是还没有写入WAL的，此时这类writer则需要选择一个leader然后继续写入WAL.
void WriteThread::ExitAsBatchGroupLeader(WriteGroup& write_group, Status status) { Writer* leader = write_group.leader; Writer* last_writer = write_group.last_writer; assert(leader->link_older == nullptr); ..................................... if (enable_pipelined_write_) { // Notify writers don't write to memtable to exit. ...................................... // Link the ramaining of the group to memtable writer list. if (write_group.size > 0) { if (LinkGroup(write_group, &newest_memtable_writer_)) { // The leader can now be different from current writer. SetState(write_group.leader, STATE_MEMTABLE_WRITER_LEADER); } } // Reset newest_writer_ and wake up the next leader. Writer* newest_writer = last_writer; if (!newest_writer_.compare_exchange_strong(newest_writer, nullptr)) { Writer* next_leader = newest_writer; while (next_leader->link_older != last_writer) { next_leader = next_leader->link_older; assert(next_leader != nullptr); } next_leader->link_older = nullptr; SetState(next_leader, STATE_GROUP_LEADER); } AwaitState(leader, STATE_MEMTABLE_WRITER_LEADER | STATE_PARALLEL_MEMTABLE_WRITER | STATE_COMPLETED, &eabgl_ctx); } else { ..................................... } } - 接下来我们来看写入memtable的操作，这里逻辑类似写入WAL,如果是leader的话，则依旧会创建一个group(WriteGroup),然后遍历需要写入memtable的writer,将他们都加入到group中(EnterAsMemTableWriter),然后则设置并发执行的大小，以及设置对应状态(LaunchParallelMemTableWriters).这里注意每次setstate就将会唤醒之前阻塞的Writer.
void WriteThread::LaunchParallelMemTableWriters(WriteGroup* write_group) { assert(write_group != nullptr); write_group->running.store(write_group->size); for (auto w : *write_group) { SetState(w, STATE_PARALLEL_MEMTABLE_WRITER); } } - 这里要注意，在构造memtable的group的时候，我们不需要创建link_newer，因为之前在写入WAL的时候，我们已经构造好link_newer,那么此时我们使用构造好的group也就是表示这个group中包含的都是已经写入到WAL的操作.
void WriteThread::EnterAsMemTableWriter(Writer* leader, WriteGroup* write_group) { .................................... if (!allow_concurrent_memtable_write_ || !leader->batch->HasMerge()) { .................................................... } write_group->last_writer = last_writer; write_group->last_sequence = last_writer->sequence + WriteBatchInternal::Count(last_writer->batch) - 1; } - 最后开始执行写入MemTable的操作，之前在写入WAL的时候被阻塞的所有Writer此时都会进入下面这个逻辑，此时也就意味着 并发写入MemTable．
if (w.state == WriteThread::STATE_PARALLEL_MEMTABLE_WRITER) { ．．．．．．．．．．．．．．．．．．．．．．．．． w.status = WriteBatchInternal::InsertInto( &w, w.sequence, &column_family_memtables, &flush_scheduler_, write_options.ignore_missing_column_families, 0 /*log_number*/, this, true /*concurrent_memtable_writes*/); if (write_thread_.CompleteParallelMemTableWriter(&w)) { MemTableInsertStatusCheck(w.status); versions_->SetLastSequence(w.write_group->last_sequence); write_thread_.ExitAsMemTableWriter(&w, *w.write_group); } } - 最后当当前group的所有Writer都写入MemTable之后，则将会调用ExitAsMemTableWriter来进行收尾工作.如果有新的memtable writer list需要处理，那么则唤醒对应的Writer,然后设置已经处理完毕的Writer的状态.
void WriteThread::ExitAsMemTableWriter(Writer* /*self*/, WriteGroup& write_group) { Writer* leader = write_group.leader; Writer* last_writer = write_group.last_writer; Writer* newest_writer = last_writer; if (!newest_memtable_writer_.compare_exchange_strong(newest_writer, nullptr)) { CreateMissingNewerLinks(newest_writer); Writer* next_leader = last_writer->link_newer; assert(next_leader != nullptr); next_leader->link_older = nullptr; SetState(next_leader, STATE_MEMTABLE_WRITER_LEADER); } Writer* w = leader; while (true) { if (!write_group.status.ok()) { w->status = write_group.status; } Writer* next = w->link_newer; if (w != leader) { SetState(w, STATE_COMPLETED); } if (w == last_writer) { break; } w = next; } // Note that leader has to exit last, since it owns the write group. SetState(leader, STATE_COMPLETED); }
总结
我们可以看到在RocksDB中，WAL的写入始终是串行写入，而MemTable可以多线程并发写入，也就是说在系统压力到一定阶段的时候， 写入WAL肯定会成为瓶颈.
