806e264350
Summary: Rocks accumulates recent writes and deletes in the in-memory memtable. When the memtable is full, it writes the contents on the memtable to a file in L0. This patch removes redundant records at the time of the flush. If there are multiple versions of the same key in the memtable, then only the most recent one is dumped into the output file. The purging of redundant records occur only if the most recent snapshot is earlier than the earliest record in the memtable. Should we switch on this feature by default or should we keep this feature turned off in the default settings? Test Plan: Added test case to db_test.cc Reviewers: sheki, vamsi, emayanke, heyongqiang Reviewed By: sheki CC: leveldb Differential Revision: https://reviews.facebook.net/D8991
89 lines
2.3 KiB
C++
89 lines
2.3 KiB
C++
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#ifndef STORAGE_LEVELDB_DB_SNAPSHOT_H_
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#define STORAGE_LEVELDB_DB_SNAPSHOT_H_
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#include "leveldb/db.h"
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namespace leveldb {
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class SnapshotList;
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// Snapshots are kept in a doubly-linked list in the DB.
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// Each SnapshotImpl corresponds to a particular sequence number.
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class SnapshotImpl : public Snapshot {
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public:
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SequenceNumber number_; // const after creation
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private:
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friend class SnapshotList;
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// SnapshotImpl is kept in a doubly-linked circular list
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SnapshotImpl* prev_;
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SnapshotImpl* next_;
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SnapshotList* list_; // just for sanity checks
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};
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class SnapshotList {
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public:
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SnapshotList() {
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list_.prev_ = &list_;
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list_.next_ = &list_;
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list_.number_ = 0xFFFFFFFFL; // placeholder marker, for debugging
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}
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bool empty() const { return list_.next_ == &list_; }
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SnapshotImpl* oldest() const { assert(!empty()); return list_.next_; }
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SnapshotImpl* newest() const { assert(!empty()); return list_.prev_; }
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const SnapshotImpl* New(SequenceNumber seq) {
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SnapshotImpl* s = new SnapshotImpl;
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s->number_ = seq;
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s->list_ = this;
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s->next_ = &list_;
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s->prev_ = list_.prev_;
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s->prev_->next_ = s;
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s->next_->prev_ = s;
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return s;
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}
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void Delete(const SnapshotImpl* s) {
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assert(s->list_ == this);
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s->prev_->next_ = s->next_;
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s->next_->prev_ = s->prev_;
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delete s;
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}
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// retrieve all snapshot numbers. They are sorted in ascending order.
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void getAll(std::vector<SequenceNumber>& ret) {
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SnapshotImpl* s = &list_;
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if (empty()) return;
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SequenceNumber prev __attribute__((unused)) = 0;
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while (s->next_ != &list_) {
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assert(prev <= s->next_->number_);
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assert(prev = s->next_->number_); // assignment
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ret.push_back(s->next_->number_);
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s = s ->next_;
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}
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}
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// get the sequence number of the most recent snapshot
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const SequenceNumber GetNewest() {
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if (empty()) {
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return 0;
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}
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return newest()->number_;
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}
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private:
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// Dummy head of doubly-linked list of snapshots
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SnapshotImpl list_;
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};
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} // namespace leveldb
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#endif // STORAGE_LEVELDB_DB_SNAPSHOT_H_
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