本质是一种归并排序算法。
这样设计的好处主要是可以减少compaction的次数和每次的文件个数。
Compaction compaction可以提高数据的查询效率，没有经过compaction，需要从很多SST file去查找，而做过compaction后，只需要从有限的SST文件去查找，大大的提高了随机查询的效率，另外也可以删除过期数据。
1. database open的时候
2. write的时候
3. read的时候？
<db/dbimpl.cc>
//是否要进行compaction
void DBImpl::MaybeScheduleCompaction() {
mutex_.AssertHeld();
if (bg_compaction_scheduled_) { //已经在进行
} else if (shutting_down_.Acquire_Load()) {
} else if (imm_ == NULL &&
manual_compaction_ == NULL &&
!versions_->NeedsCompaction()) {
//imm_为NULL:没有memtable需要flush
//manual_compaction_:手动compaction
} else {
bg_compaction_scheduled_ = true;
env_->Schedule(&DBImpl::BGWork, this);
}
}
<db/version_set.h>
bool NeedsCompaction() const {
Version* v = current_;
return (v->compaction_score_ >= 1) || (v->file_to_compact_ != NULL);
}
如何计算这个compaction_score呢？看下面的代码：
<db/version_set.cc>
void VersionSet::Finalize(Version* v) {
int best_level = -1;
double best_score = -1;
//遍历所有的level
for (int level = 0; level < config::kNumLevels-1; level++) {
double score;
if (level == 0) {
//对于level 0,计算当前文件个数和预定义的compaction trigger value(Default:4)之比
score = v->files_[level].size() /
static_cast<double>(config::kL0_CompactionTrigger);
} else {
//对于其他level,计算level文件大小和level应有的大小(10^N MB)
const uint64_t level_bytes = TotalFileSize(v->files_[level]);
score = static_cast<double>(level_bytes) / MaxBytesForLevel(level);
}
//找出最需要compaction的level
if (score > best_score) {
best_level = level;
best_score = score;
}
}
v->compaction_level_ = best_level;
v->compaction_score_ = best_score;
}
leveldb 运行会启动一个background thread,会执行一些background task，compaction就在这个线程中执行。
首先来看看compaction对象如何定义的
<db/version_set.h>
//关于compaction的一些信息
class Compaction {
public:
~Compaction();
//compaction Level:会将N层N+1层合并生成N+1文件
int level() const { return level_; }
//返回VersionEdit,用于记录到manifest
VersionEdit* edit() { return &edit_; }
//返回N层或者N+1层的文件个数，which = 0,1
int num_input_files(int which) const { return inputs_[which].size(); }
//返回具体的文件信息,which:level
FileMetaData* input(int which, int i) const { return inputs_[which][i]; }
//本次compaction最大输出字节
uint64_t MaxOutputFileSize() const { return max_output_file_size_; }
//是否只需要移动文件进行compaction,不需要merge和split
bool IsTrivialMove() const;
//把input都当成delete写入edit
void AddInputDeletions(VersionEdit* edit);
// Returns true if the information we have available guarantees that
// the compaction is producing data in "level+1" for which no data exists
// in levels greater than "level+1".
bool IsBaseLevelForKey(const Slice& user_key);
// Returns true iff we should stop building the current output
// before processing "internal_key".
bool ShouldStopBefore(const Slice& internal_key);
// Release the input version for the compaction, once the compaction
// is successful.
void ReleaseInputs();
private:
friend class Version;
friend class VersionSet;
explicit Compaction(int level);
int level_;
uint64_t max_output_file_size_;
Version* input_version_;
VersionEdit edit_;
// Each compaction reads inputs from "level_" and "level_+1"
std::vector<FileMetaData*> inputs_[2]; // The two sets of inputs
// State used to check for number of of overlapping grandparent files
// (parent == level_ + 1, grandparent == level_ + 2)
std::vector<FileMetaData*> grandparents_;
size_t grandparent_index_; // Index in grandparent_starts_
bool seen_key_; // Some output key has been seen
int64_t overlapped_bytes_; // Bytes of overlap between current output
// and grandparent files
// State for implementing IsBaseLevelForKey
// level_ptrs_ holds indices into input_version_->levels_: our state
// is that we are positioned at one of the file ranges for each
// higher level than the ones involved in this compaction (i.e. for
// all L >= level_ + 2).
size_t level_ptrs_[config::kNumLevels];
};
Compaction Thread
<db/dbimpl.cc>
void DBImpl::BackgroundCompaction() {
mutex_.AssertHeld();
//把memtable flush到sstable
if (imm_ != NULL) {
CompactMemTable();
return;
}
Compaction* c;
bool is_manual = (manual_compaction_ != NULL);
InternalKey manual_end;
if (is_manual) { //手动compaction
ManualCompaction* m = manual_compaction_;
//根据range来做compaction
c = versions_->CompactRange(m->level, m->begin, m->end);
m->done = (c == NULL);
if (c != NULL) {
manual_end = c->input(0, c->num_input_files(0) - 1)->largest;
}
Log(options_.info_log,
"Manual compaction at level-%d from %s .. %s; will stop at %s\n",
m->level,
(m->begin ? m->begin->DebugString().c_str() : "(begin)"),
(m->end ? m->end->DebugString().c_str() : "(end)"),
(m->done ? "(end)" : manual_end.DebugString().c_str()));
} else {
//找到需要compaction的level&file
c = versions_->PickCompaction();
}
Status status;
if (c == NULL) {
// Nothing to do
} else if (!is_manual && c->IsTrivialMove()) { //只需要移动sst file
// Move file to next level
assert(c->num_input_files(0) == 1);
FileMetaData* f = c->input(0, 0);
c->edit()->DeleteFile(c->level(), f->number);
c->edit()->AddFile(c->level() + 1, f->number, f->file_size,
f->smallest, f->largest);
status = versions_->LogAndApply(c->edit(), &mutex_);
VersionSet::LevelSummaryStorage tmp;
Log(options_.info_log, "Moved #%lld to level-%d %lld bytes %s: %s\n",
static_cast<unsigned long long>(f->number),
c->level() + 1,
static_cast<unsigned long long>(f->file_size),
status.ToString().c_str(),
versions_->LevelSummary(&tmp));
} else {//完成compaction
CompactionState* compact = new CompactionState(c);
status = DoCompactionWork(compact);
CleanupCompaction(compact);
c->ReleaseInputs();
DeleteObsoleteFiles();
}
delete c;
if (status.ok()) {
// Done
} else if (shutting_down_.Acquire_Load()) {
// Ignore compaction errors found during shutting down
} else {
Log(options_.info_log,
"Compaction error: %s", status.ToString().c_str());
if (options_.paranoid_checks && bg_error_.ok()) {
bg_error_ = status;
}
}
if (is_manual) {
ManualCompaction* m = manual_compaction_;
if (!status.ok()) {
m->done = true;
}
if (!m->done) {
// We only compacted part of the requested range. Update *m
// to the range that is left to be compacted.
m->tmp_storage = manual_end;
m->begin = &m->tmp_storage;
}
manual_compaction_ = NULL;
}
}
compaction memtable:写一个level0文件，并写入manifest log
Status DBImpl::CompactMemTable() {
mutex_.AssertHeld();
assert(imm_ != NULL);
VersionEdit edit;
Version* base = versions_->current();
base->Ref();
//写入level0 sst table
Status s = WriteLevel0Table(imm_, &edit, base);
base->Unref();
if (s.ok() && shutting_down_.Acquire_Load()) {
s = Status::IOError("Deleting DB during memtable compaction");
}
// Replace immutable memtable with the generated Table
if (s.ok()) {
edit.SetPrevLogNumber(0);
edit.SetLogNumber(logfile_number_); // Earlier logs no longer needed
//生成edit并计入manifest log
s = versions_->LogAndApply(&edit, &mutex_);
}
if (s.ok()) {
// Commit to the new state
imm_->Unref();
imm_ = NULL;
has_imm_.Release_Store(NULL);
DeleteObsoleteFiles();
}
return s;
}
下面来看看compaction已有文件：
找出要compaction的文件:
<db/version_set.cc>
Compaction* VersionSet::PickCompaction() {
Compaction* c;
int level;
//是否需要compaction,有两种compaction，一种基于size大小，另外一种基于被seek的次数
const bool size_compaction = (current_->compaction_score_ >= 1);
const bool seek_compaction = (current_->file_to_compact_ != NULL);
if (size_compaction) {
level = current_->compaction_level_;
assert(level >= 0);
assert(level+1 < config::kNumLevels);
c = new Compaction(level);
//每一层有一个compact_pointer，用于记录compaction key，这样可以进行循环compaction
for (size_t i = 0; i < current_->files_[level].size(); i++) {
FileMetaData* f = current_->files_[level][i];
if (compact_pointer_[level].empty() ||
icmp_.Compare(f->largest.Encode(), compact_pointer_[level]) > 0) {
//找到一个文件就可以了
c->inputs_[0].push_back(f);
break;
}
}
if (c->inputs_[0].empty()) {
// Wrap-around to the beginning of the key space
c->inputs_[0].push_back(current_->files_[level][0]);
}
} else if (seek_compaction) {
level = current_->file_to_compact_level_;
c = new Compaction(level);
c->inputs_[0].push_back(current_->file_to_compact_);
} else {
return NULL;
}
c->input_version_ = current_;
c->input_version_->Ref();
// level 0：特殊处理，因为可能有key 重叠，把所有重叠都找出来，一起做compaction
if (level == 0) {
InternalKey smallest, largest;
GetRange(c->inputs_[0], &smallest, &largest);
// Note that the next call will discard the file we placed in
// c->inputs_[0] earlier and replace it with an overlapping set
// which will include the picked file.
current_->GetOverlappingInputs(0, &smallest, &largest, &c->inputs_[0]);
assert(!c->inputs_[0].empty());
}
//找到level N+1需要compaction的文件
SetupOtherInputs(c);
return c;
}
<db/version_set.cc>
void VersionSet::SetupOtherInputs(Compaction* c) {
const int level = c->level();
InternalKey smallest, largest;
GetRange(c->inputs_[0], &smallest, &largest);
//找到所有在Level N+1层有重叠的文件
current_->GetOverlappingInputs(level+1, &smallest, &largest, &c->inputs_[1]);
//取出key的范围
InternalKey all_start, all_limit;
GetRange2(c->inputs_[0], c->inputs_[1], &all_start, &all_limit);
//检查是否能从Level N找到更多的文件
if (!c->inputs_[1].empty()) {
std::vector<FileMetaData*> expanded0;
current_->GetOverlappingInputs(level, &all_start, &all_limit, &expanded0);
const int64_t inputs0_size = TotalFileSize(c->inputs_[0]);
const int64_t inputs1_size = TotalFileSize(c->inputs_[1]);
const int64_t expanded0_size = TotalFileSize(expanded0);
if (expanded0.size() > c->inputs_[0].size() &&
inputs1_size + expanded0_size < kExpandedCompactionByteSizeLimit) {
InternalKey new_start, new_limit;
GetRange(expanded0, &new_start, &new_limit);
std::vector<FileMetaData*> expanded1;
current_->GetOverlappingInputs(level+1, &new_start, &new_limit,
&expanded1);
if (expanded1.size() == c->inputs_[1].size()) {
Log(options_->info_log,
"Expanding@%d %d+%d (%ld+%ld bytes) to %d+%d (%ld+%ld bytes)\n",
level,
int(c->inputs_[0].size()),
int(c->inputs_[1].size()),
long(inputs0_size), long(inputs1_size),
int(expanded0.size()),
int(expanded1.size()),
long(expanded0_size), long(inputs1_size));
smallest = new_start;
largest = new_limit;
c->inputs_[0] = expanded0;
c->inputs_[1] = expanded1;
GetRange2(c->inputs_[0], c->inputs_[1], &all_start, &all_limit);
}
}
}
// Compute the set of grandparent files that overlap this compaction
// (parent == level+1; grandparent == level+2)
if (level + 2 < config::kNumLevels) {
current_->GetOverlappingInputs(level + 2, &all_start, &all_limit,
&c->grandparents_);
}
if (false) {
Log(options_->info_log, "Compacting %d '%s' .. '%s'",
level,
smallest.DebugString().c_str(),
largest.DebugString().c_str());
}
//设置新的compact_pointer
compact_pointer_[level] = largest.Encode().ToString();
c->edit_.SetCompactPointer(level, largest);
}
do compaction task:
Status DBImpl::DoCompactionWork(CompactionState* compact) {
const uint64_t start_micros = env_->NowMicros();
int64_t imm_micros = 0; // Micros spent doing imm_ compactions
Log(options_.info_log, "Compacting %d@%d + %d@%d files",
compact->compaction->num_input_files(0),
compact->compaction->level(),
compact->compaction->num_input_files(1),
compact->compaction->level() + 1);
assert(versions_->NumLevelFiles(compact->compaction->level()) > 0);
assert(compact->builder == NULL);
assert(compact->outfile == NULL);
if (snapshots_.empty()) {
compact->smallest_snapshot = versions_->LastSequence();
} else {
compact->smallest_snapshot = snapshots_.oldest()->number_;
}
// Release mutex while we're actually doing the compaction work
mutex_.Unlock();
//生成iterator:遍历要compaction的数据
Iterator* input = versions_->MakeInputIterator(compact->compaction);
input->SeekToFirst();
Status status;
ParsedInternalKey ikey;
std::string current_user_key;
bool has_current_user_key = false;
SequenceNumber last_sequence_for_key = kMaxSequenceNumber;
for (; input->Valid() && !shutting_down_.Acquire_Load(); ) {
// 如果有memtable要compaction:优先去做
if (has_imm_.NoBarrier_Load() != NULL) {
const uint64_t imm_start = env_->NowMicros();
mutex_.Lock();
if (imm_ != NULL) {
CompactMemTable();
bg_cv_.SignalAll(); // Wakeup MakeRoomForWrite() if necessary
}
mutex_.Unlock();
imm_micros += (env_->NowMicros() - imm_start);
}
Slice key = input->key();
//检查是不是中途输出compaction的结果，避免compaction结果和level N+2 files有过多的重叠
if (compact->compaction->ShouldStopBefore(key) &&
compact->builder != NULL) {
status = FinishCompactionOutputFile(compact, input);
if (!status.ok()) {
break;
}
}
// Handle key/value, add to state, etc.
bool drop = false;
if (!ParseInternalKey(key, &ikey)) {
// Do not hide error keys
current_user_key.clear();
has_current_user_key = false;
last_sequence_for_key = kMaxSequenceNumber;
} else {
if (!has_current_user_key ||
user_comparator()->Compare(ikey.user_key,
Slice(current_user_key)) != 0) {
// First occurrence of this user key
current_user_key.assign(ikey.user_key.data(), ikey.user_key.size());
has_current_user_key = true;
last_sequence_for_key = kMaxSequenceNumber;
}
if (last_sequence_for_key <= compact->smallest_snapshot) {
// Hidden by an newer entry for same user key
drop = true; // (A)
} else if (ikey.type == kTypeDeletion &&
ikey.sequence <= compact->smallest_snapshot &&
compact->compaction->IsBaseLevelForKey(ikey.user_key)) {
// For this user key:
// (1) there is no data in higher levels
// (2) data in lower levels will have larger sequence numbers
// (3) data in layers that are being compacted here and have
// smaller sequence numbers will be dropped in the next
// few iterations of this loop (by rule (A) above).
// Therefore this deletion marker is obsolete and can be dropped.
drop = true;
}
last_sequence_for_key = ikey.sequence;
}
if (!drop) {
// Open output file if necessary
if (compact->builder == NULL) {
status = OpenCompactionOutputFile(compact);
if (!status.ok()) {
break;
}
}
if (compact->builder->NumEntries() == 0) {
compact->current_output()->smallest.DecodeFrom(key);
}
compact->current_output()->largest.DecodeFrom(key);
compact->builder->Add(key, input->value());
// 达到sst文件大小，重新写文件
if (compact->builder->FileSize() >=
compact->compaction->MaxOutputFileSize()) {
status = FinishCompactionOutputFile(compact, input);
if (!status.ok()) {
break;
}
}
}
input->Next();
}
if (status.ok() && shutting_down_.Acquire_Load()) {
status = Status::IOError("Deleting DB during compaction");
}
if (status.ok() && compact->builder != NULL) {
status = FinishCompactionOutputFile(compact, input);
}
if (status.ok()) {
status = input->status();
}
delete input;
input = NULL;
//更新compaction的一些统计数据
CompactionStats stats;
stats.micros = env_->NowMicros() - start_micros - imm_micros;
for (int which = 0; which < 2; which++) {
for (int i = 0; i < compact->compaction->num_input_files(which); i++) {
stats.bytes_read += compact->compaction->input(which, i)->file_size;
}
}
for (size_t i = 0; i < compact->outputs.size(); i++) {
stats.bytes_written += compact->outputs[i].file_size;
}
mutex_.Lock();
stats_[compact->compaction->level() + 1].Add(stats);
if (status.ok()) {
status = InstallCompactionResults(compact);
}
VersionSet::LevelSummaryStorage tmp;
Log(options_.info_log,
"compacted to: %s", versions_->LevelSummary(&tmp));
return status;
}