rocksdb/db/compaction_job.cc

//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
//  This source code is licensed under the BSD-style license found in the
//  LICENSE file in the root directory of this source tree. An additional grant
//  of patent rights can be found in the PATENTS file in the same directory.
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#include "db/compaction_job.h"

#ifndef __STDC_FORMAT_MACROS
#define __STDC_FORMAT_MACROS
#endif

#include <inttypes.h>
#include <algorithm>
#include <vector>
#include <memory>
#include <list>

#include "db/builder.h"
#include "db/db_iter.h"
#include "db/dbformat.h"
#include "db/event_helpers.h"
#include "db/filename.h"
#include "db/log_reader.h"
#include "db/log_writer.h"
#include "db/memtable.h"
#include "db/merge_helper.h"
#include "db/memtable_list.h"
#include "db/merge_context.h"
#include "db/version_set.h"
#include "port/port.h"
#include "port/likely.h"
#include "rocksdb/db.h"
#include "rocksdb/env.h"
#include "rocksdb/statistics.h"
#include "rocksdb/status.h"
#include "rocksdb/table.h"
#include "table/block.h"
#include "table/block_based_table_factory.h"
#include "table/merger.h"
#include "table/table_builder.h"
#include "table/two_level_iterator.h"
#include "util/coding.h"
#include "util/file_reader_writer.h"
#include "util/logging.h"
#include "util/log_buffer.h"
#include "util/mutexlock.h"
#include "util/perf_context_imp.h"
#include "util/iostats_context_imp.h"
#include "util/stop_watch.h"
#include "util/string_util.h"
#include "util/sync_point.h"
#include "util/thread_status_util.h"

namespace rocksdb {

// Maintains state for each sub-compaction
struct CompactionJob::SubCompactionState {
  Compaction* compaction;

  // The boundaries of the key-range this compaction is interested in. No two
  // subcompactions may have overlapping key-ranges.
  // 'start' is inclusive, 'end' is exclusive, and nullptr means unbounded
  Slice *start, *end;

  // The return status of this compaction
  Status status;

  // Files produced by compaction
  struct Output {
    uint64_t number;
    uint32_t path_id;
    uint64_t file_size;
    InternalKey smallest, largest;
    SequenceNumber smallest_seqno, largest_seqno;
    bool need_compaction;
  };

  // State kept for output being generated
  std::vector<Output> outputs;
  std::unique_ptr<WritableFileWriter> outfile;
  std::unique_ptr<TableBuilder> builder;
  Output* current_output() {
    if (outputs.empty()) {
      // This subcompaction's ouptut could be empty if compaction was aborted
      // before this subcompaction had a chance to generate any output files.
      // When subcompactions are executed sequentially this is more likely and
      // will be particulalry likely for the last subcompaction to be empty.
      // Once they are run in parallel however it should be much rarer.
      return nullptr;
    } else {
      return &outputs.back();
    }
  }

  // State during the sub-compaction
  uint64_t total_bytes;
  uint64_t num_input_records;
  uint64_t num_output_records;
  CompactionJobStats compaction_job_stats;

  // "level_ptrs" holds indices that remember which file of an associated
  // level we were last checking during the last call to compaction->
  // KeyNotExistsBeyondOutputLevel(). This allows future calls to the function
  // to pick off where it left off since each subcompaction's key range is
  // increasing so a later call to the function must be looking for a key that
  // is in or beyond the last file checked during the previous call
  std::vector<size_t> level_ptrs;

  SubCompactionState(Compaction* c, Slice* _start, Slice* _end)
      : compaction(c),
        start(_start),
        end(_end),
        outfile(nullptr),
        builder(nullptr),
        total_bytes(0),
        num_input_records(0),
        num_output_records(0) {
    assert(compaction != nullptr);
    level_ptrs = std::vector<size_t>(compaction->number_levels(), 0);
  }

  SubCompactionState(SubCompactionState&& o) {
    *this = std::move(o);
  }

  SubCompactionState& operator=(SubCompactionState&& o) {
    compaction = std::move(o.compaction);
    start = std::move(o.start);
    end = std::move(o.end);
    status = std::move(o.status);
    outputs = std::move(o.outputs);
    outfile = std::move(o.outfile);
    builder = std::move(o.builder);
    total_bytes = std::move(o.total_bytes);
    num_input_records = std::move(o.num_input_records);
    num_output_records = std::move(o.num_output_records);
    level_ptrs = std::move(o.level_ptrs);
    return *this;
  }

  // Because member unique_ptrs do not have these.
  SubCompactionState(const SubCompactionState&) = delete;

  SubCompactionState& operator=(const SubCompactionState&) = delete;
};

// Maintains state for the entire compaction
struct CompactionJob::CompactionState {
  Compaction* const compaction;

  // REQUIRED: subcompaction states are stored in order of increasing
  // key-range
  std::vector<CompactionJob::SubCompactionState> sub_compact_states;
  Status status;

  uint64_t total_bytes;
  uint64_t num_input_records;
  uint64_t num_output_records;

  explicit CompactionState(Compaction* c)
      : compaction(c),
        total_bytes(0),
        num_input_records(0),
        num_output_records(0) {}

  size_t NumOutputFiles() {
    size_t total = 0;
    for (auto& s : sub_compact_states) {
      total += s.outputs.size();
    }
    return total;
  }

  Slice SmallestUserKey() {
    for (size_t i = 0; i < sub_compact_states.size(); i++) {
      if (!sub_compact_states[i].outputs.empty()) {
        return sub_compact_states[i].outputs[0].smallest.user_key();
      }
    }
    // TODO(aekmekji): should we exit with an error if it reaches here?
    assert(0);
    return Slice(nullptr, 0);
  }

  Slice LargestUserKey() {
    for (int i = static_cast<int>(sub_compact_states.size() - 1); i >= 0; i--) {
      if (!sub_compact_states[i].outputs.empty()) {
        assert(sub_compact_states[i].current_output() != nullptr);
        return sub_compact_states[i].current_output()->largest.user_key();
      }
    }
    // TODO(aekmekji): should we exit with an error if it reaches here?
    assert(0);
    return Slice(nullptr, 0);
  }
};

void CompactionJob::AggregateStatistics() {
  for (SubCompactionState& sc : compact_->sub_compact_states) {
    compact_->total_bytes += sc.total_bytes;
    compact_->num_input_records += sc.num_input_records;
    compact_->num_output_records += sc.num_output_records;

    if (compaction_job_stats_) {
      compaction_job_stats_->Add(sc.compaction_job_stats);
    }
  }
}

CompactionJob::CompactionJob(
    int job_id, Compaction* compaction, const DBOptions& db_options,
    const EnvOptions& env_options, VersionSet* versions,
    std::atomic<bool>* shutting_down, LogBuffer* log_buffer,
    Directory* db_directory, Directory* output_directory, Statistics* stats,
    std::vector<SequenceNumber> existing_snapshots,
    std::shared_ptr<Cache> table_cache, EventLogger* event_logger,
    bool paranoid_file_checks, bool measure_io_stats, const std::string& dbname,
    CompactionJobStats* compaction_job_stats)
    : job_id_(job_id),
      compact_(new CompactionState(compaction)),
      compaction_job_stats_(compaction_job_stats),
      compaction_stats_(1),
      dbname_(dbname),
      db_options_(db_options),
      env_options_(env_options),
      env_(db_options.env),
      versions_(versions),
      shutting_down_(shutting_down),
      log_buffer_(log_buffer),
      db_directory_(db_directory),
      output_directory_(output_directory),
      stats_(stats),
      existing_snapshots_(std::move(existing_snapshots)),
      table_cache_(std::move(table_cache)),
      event_logger_(event_logger),
      paranoid_file_checks_(paranoid_file_checks),
      measure_io_stats_(measure_io_stats) {
  assert(log_buffer_ != nullptr);
  ThreadStatusUtil::SetColumnFamily(compact_->compaction->column_family_data());
  ThreadStatusUtil::SetThreadOperation(ThreadStatus::OP_COMPACTION);
  ReportStartedCompaction(compaction);
}

CompactionJob::~CompactionJob() {
  assert(compact_ == nullptr);
  ThreadStatusUtil::ResetThreadStatus();
}

void CompactionJob::ReportStartedCompaction(
    Compaction* compaction) {
  ThreadStatusUtil::SetColumnFamily(
      compact_->compaction->column_family_data());

  ThreadStatusUtil::SetThreadOperationProperty(
      ThreadStatus::COMPACTION_JOB_ID,
      job_id_);

  ThreadStatusUtil::SetThreadOperationProperty(
      ThreadStatus::COMPACTION_INPUT_OUTPUT_LEVEL,
      (static_cast<uint64_t>(compact_->compaction->start_level()) << 32) +
          compact_->compaction->output_level());

  // In the current design, a CompactionJob is always created
  // for non-trivial compaction.
  assert(compaction->IsTrivialMove() == false ||
         compaction->is_manual_compaction() == true);

  ThreadStatusUtil::SetThreadOperationProperty(
      ThreadStatus::COMPACTION_PROP_FLAGS,
      compaction->is_manual_compaction() +
          (compaction->deletion_compaction() << 1));

  ThreadStatusUtil::SetThreadOperationProperty(
      ThreadStatus::COMPACTION_TOTAL_INPUT_BYTES,
      compaction->CalculateTotalInputSize());

  IOSTATS_RESET(bytes_written);
  IOSTATS_RESET(bytes_read);
  ThreadStatusUtil::SetThreadOperationProperty(
      ThreadStatus::COMPACTION_BYTES_WRITTEN, 0);
  ThreadStatusUtil::SetThreadOperationProperty(
      ThreadStatus::COMPACTION_BYTES_READ, 0);

  // Set the thread operation after operation properties
  // to ensure GetThreadList() can always show them all together.
  ThreadStatusUtil::SetThreadOperation(
      ThreadStatus::OP_COMPACTION);

  if (compaction_job_stats_) {
    compaction_job_stats_->is_manual_compaction =
        compaction->is_manual_compaction();
  }
}

void CompactionJob::Prepare() {
  AutoThreadOperationStageUpdater stage_updater(
      ThreadStatus::STAGE_COMPACTION_PREPARE);

  // Generate file_levels_ for compaction berfore making Iterator
  auto* c = compact_->compaction;
  assert(c->column_family_data() != nullptr);
  assert(c->column_family_data()->current()->storage_info()
      ->NumLevelFiles(compact_->compaction->level()) > 0);

  // Is this compaction producing files at the bottommost level?
  bottommost_level_ = c->bottommost_level();

  // Initialize subcompaction states
  latest_snapshot_ = 0;
  visible_at_tip_ = 0;

  if (existing_snapshots_.size() == 0) {
    // optimize for fast path if there are no snapshots
    visible_at_tip_ = versions_->LastSequence();
    earliest_snapshot_ = visible_at_tip_;
  } else {
    latest_snapshot_ = existing_snapshots_.back();
    // Add the current seqno as the 'latest' virtual
    // snapshot to the end of this list.
    existing_snapshots_.push_back(versions_->LastSequence());
    earliest_snapshot_ = existing_snapshots_[0];
  }

  InitializeSubCompactions();
}

// For L0-L1 compaction, iterators work in parallel by processing
// different subsets of the full key range. This function sets up
// the local states used by each of these subcompactions during
// their execution
void CompactionJob::InitializeSubCompactions() {
  Compaction* c = compact_->compaction;
  auto& bounds = sub_compaction_boundaries_;
  if (c->IsSubCompaction()) {
    auto* cmp = c->column_family_data()->user_comparator();
    for (size_t which = 0; which < c->num_input_levels(); which++) {
      if (c->level(which) == 1) {
        const LevelFilesBrief* flevel = c->input_levels(which);
        size_t num_files = flevel->num_files;

        if (num_files > 1) {
          std::vector<Slice> candidates;
          auto& files = flevel->files;
          Slice global_min = ExtractUserKey(files[0].smallest_key);
          Slice global_max = ExtractUserKey(files[num_files - 1].largest_key);

          for (size_t i = 1; i < num_files; i++) {
            // Make sure the smallest key in two consecutive L1 files are
            // unique before adding the smallest key as a boundary. Also ensure
            // that the boundary won't lead to an empty subcompaction (happens
            // if the boundary == the smallest or largest key)
            Slice s1 = ExtractUserKey(files[i].smallest_key);
            Slice s2 = i == num_files - 1
                          ? Slice()
                          : ExtractUserKey(files[i + 1].smallest_key);

            if ( (i == num_files - 1 && cmp->Compare(s1, global_max) < 0)
              || (i < num_files - 1 && cmp->Compare(s1, s2) < 0 &&
                    cmp->Compare(s1, global_min) > 0)) {
              candidates.emplace_back(s1);
            }
          }

          // Divide the potential L1 file boundaries (those that passed the
          // checks above) into 'max_subcompactions' groups such that each have
          // as close to an equal number of files in it as possible
          // TODO(aekmekji): refine this later to depend on file size
          size_t files_left = candidates.size();
          size_t subcompactions_left =
              static_cast<size_t>(db_options_.max_subcompactions) < files_left
                  ? db_options_.max_subcompactions
                  : files_left;

          size_t num_to_include;
          size_t index = 0;

          while (files_left > 1 && subcompactions_left > 1) {
            num_to_include = files_left / subcompactions_left;
            index += num_to_include;
            sub_compaction_boundaries_.emplace_back(candidates[index]);
            files_left -= num_to_include;
            subcompactions_left--;
          }
        }
        break;
      }
    }
  }

  // Note: it's necessary for the first iterator sub-range to have
  // start == nullptr and for the last to have end == nullptr
  for (size_t i = 0; i <= bounds.size(); i++) {
    Slice *start = i == 0 ? nullptr : &bounds[i - 1];
    Slice *end = i == bounds.size() ? nullptr : &bounds[i];
    compact_->sub_compact_states.emplace_back(compact_->compaction, start, end);
  }
}

Status CompactionJob::Run() {
  AutoThreadOperationStageUpdater stage_updater(
      ThreadStatus::STAGE_COMPACTION_RUN);
  TEST_SYNC_POINT("CompactionJob::Run():Start");
  log_buffer_->FlushBufferToLog();
  LogCompaction();

  // Run each subcompaction sequentially
  const uint64_t start_micros = env_->NowMicros();
  for (size_t i = 0; i < compact_->sub_compact_states.size(); i++) {
    ProcessKeyValueCompaction(&compact_->sub_compact_states[i]);
  }
  compaction_stats_.micros = env_->NowMicros() - start_micros;
  MeasureTime(stats_, COMPACTION_TIME, compaction_stats_.micros);

  // Determine if any of the subcompactions failed
  Status status;
  for (const auto& state : compact_->sub_compact_states) {
    if (!state.status.ok()) {
      status = state.status;
      break;
    }
  }

  // Finish up all book-keeping to unify the subcompaction results
  AggregateStatistics();
  UpdateCompactionStats();
  RecordCompactionIOStats();
  LogFlush(db_options_.info_log);
  TEST_SYNC_POINT("CompactionJob::Run():End");

  compact_->status = status;
  return status;
}

Status CompactionJob::Install(const MutableCFOptions& mutable_cf_options,
                              InstrumentedMutex* db_mutex) {
  AutoThreadOperationStageUpdater stage_updater(
      ThreadStatus::STAGE_COMPACTION_INSTALL);
  db_mutex->AssertHeld();
  Status status = compact_->status;
  ColumnFamilyData* cfd = compact_->compaction->column_family_data();
  cfd->internal_stats()->AddCompactionStats(
      compact_->compaction->output_level(), compaction_stats_);

  if (status.ok()) {
    status = InstallCompactionResults(mutable_cf_options, db_mutex);
  }
  VersionStorageInfo::LevelSummaryStorage tmp;
  auto vstorage = cfd->current()->storage_info();
  const auto& stats = compaction_stats_;
  LogToBuffer(
      log_buffer_,
      "[%s] compacted to: %s, MB/sec: %.1f rd, %.1f wr, level %d, "
      "files in(%d, %d) out(%d) "
      "MB in(%.1f, %.1f) out(%.1f), read-write-amplify(%.1f) "
      "write-amplify(%.1f) %s, records in: %d, records dropped: %d\n",
      cfd->GetName().c_str(), vstorage->LevelSummary(&tmp),
      (stats.bytes_read_non_output_levels + stats.bytes_read_output_level) /
          static_cast<double>(stats.micros),
      stats.bytes_written / static_cast<double>(stats.micros),
      compact_->compaction->output_level(),
      stats.num_input_files_in_non_output_levels,
      stats.num_input_files_in_output_level,
      stats.num_output_files,
      stats.bytes_read_non_output_levels / 1048576.0,
      stats.bytes_read_output_level / 1048576.0,
      stats.bytes_written / 1048576.0,
      (stats.bytes_written + stats.bytes_read_output_level +
       stats.bytes_read_non_output_levels) /
          static_cast<double>(stats.bytes_read_non_output_levels),
      stats.bytes_written /
          static_cast<double>(stats.bytes_read_non_output_levels),
      status.ToString().c_str(), stats.num_input_records,
      stats.num_dropped_records);

  UpdateCompactionJobStats(stats);

  auto stream = event_logger_->LogToBuffer(log_buffer_);
  stream << "job" << job_id_ << "event"
         << "compaction_finished"
         << "output_level" << compact_->compaction->output_level()
         << "num_output_files" << compact_->NumOutputFiles()
         << "total_output_size" << compact_->total_bytes
         << "num_input_records" << compact_->num_input_records
         << "num_output_records" << compact_->num_output_records;

  if (measure_io_stats_ && compaction_job_stats_ != nullptr) {
    stream << "file_write_nanos" << compaction_job_stats_->file_write_nanos;
    stream << "file_range_sync_nanos"
           << compaction_job_stats_->file_range_sync_nanos;
    stream << "file_fsync_nanos" << compaction_job_stats_->file_fsync_nanos;
    stream << "file_prepare_write_nanos"
           << compaction_job_stats_->file_prepare_write_nanos;
  }

  stream << "lsm_state";
  stream.StartArray();
  for (int level = 0; level < vstorage->num_levels(); ++level) {
    stream << vstorage->NumLevelFiles(level);
  }
  stream.EndArray();

  CleanupCompaction();
  return status;
}

void CompactionJob::ProcessKeyValueCompaction(SubCompactionState* sub_compact) {
  assert(sub_compact != nullptr);
  std::unique_ptr<Iterator> input_ptr(
      versions_->MakeInputIterator(sub_compact->compaction));
  Iterator* input = input_ptr.get();

  AutoThreadOperationStageUpdater stage_updater(
      ThreadStatus::STAGE_COMPACTION_PROCESS_KV);

  // I/O measurement variables
  PerfLevel prev_perf_level = PerfLevel::kEnableTime;
  uint64_t prev_write_nanos = 0;
  uint64_t prev_fsync_nanos = 0;
  uint64_t prev_range_sync_nanos = 0;
  uint64_t prev_prepare_write_nanos = 0;
  if (measure_io_stats_) {
    prev_perf_level = GetPerfLevel();
    SetPerfLevel(PerfLevel::kEnableTime);
    prev_write_nanos = iostats_context.write_nanos;
    prev_fsync_nanos = iostats_context.fsync_nanos;
    prev_range_sync_nanos = iostats_context.range_sync_nanos;
    prev_prepare_write_nanos = iostats_context.prepare_write_nanos;
  }

  // Variables used inside the loop
  Status status;
  std::string compaction_filter_value;
  ParsedInternalKey ikey;
  IterKey current_user_key;
  bool has_current_user_key = false;
  IterKey delete_key;

  SequenceNumber last_sequence_for_key __attribute__((unused)) =
        kMaxSequenceNumber;
  SequenceNumber visible_in_snapshot = kMaxSequenceNumber;
  ColumnFamilyData* cfd = sub_compact->compaction->column_family_data();
  MergeHelper merge(cfd->user_comparator(), cfd->ioptions()->merge_operator,
                    db_options_.info_log.get(),
                    cfd->ioptions()->min_partial_merge_operands,
                    false /* internal key corruption is expected */);
  auto compaction_filter = cfd->ioptions()->compaction_filter;
  std::unique_ptr<CompactionFilter> compaction_filter_from_factory = nullptr;
  if (compaction_filter == nullptr) {
    compaction_filter_from_factory =
        sub_compact->compaction->CreateCompactionFilter();
    compaction_filter = compaction_filter_from_factory.get();
  }

  TEST_SYNC_POINT("CompactionJob::Run():Inprogress");

  int64_t key_drop_user = 0;
  int64_t key_drop_newer_entry = 0;
  int64_t key_drop_obsolete = 0;
  int64_t loop_cnt = 0;

  StopWatchNano timer(env_, stats_ != nullptr);
  uint64_t total_filter_time = 0;

  Slice* start = sub_compact->start;
  Slice* end = sub_compact->end;
  if (start != nullptr) {
    IterKey start_iter;
    start_iter.SetInternalKey(*start, kMaxSequenceNumber, kValueTypeForSeek);
    Slice start_key = start_iter.GetKey();
    input->Seek(start_key);
  } else {
    input->SeekToFirst();
  }

  // TODO(noetzli): check whether we could check !shutting_down_->... only
  // only occasionally (see diff D42687)
  while (input->Valid() && !shutting_down_->load(std::memory_order_acquire) &&
         !cfd->IsDropped() && status.ok()) {
    Slice key = input->key();
    Slice value = input->value();

    // First check that the key is parseable before performing the comparison
    // to determine if it's within the range we want. Parsing may fail if the
    // key being passed in is a user key without any internal key component
    if (!ParseInternalKey(key, &ikey)) {
      // Do not hide error keys
      // TODO: error key stays in db forever? Figure out the rationale
      // v10 error v8 : we cannot hide v8 even though it's pretty obvious.
      current_user_key.Clear();
      has_current_user_key = false;
      last_sequence_for_key = kMaxSequenceNumber;
      visible_in_snapshot = kMaxSequenceNumber;
      sub_compact->compaction_job_stats.num_corrupt_keys++;

      status = WriteKeyValue(key, value, ikey, input->status(), sub_compact);
      input->Next();
      continue;
    }

    // If an end key (exclusive) is specified, check if the current key is
    // >= than it and exit if it is because the iterator is out of its range
    if (end != nullptr &&
        cfd->user_comparator()->Compare(ikey.user_key, *end) >= 0) {
      break;
    }

    sub_compact->num_input_records++;
    if (++loop_cnt > 1000) {
      RecordDroppedKeys(&key_drop_user, &key_drop_newer_entry,
                        &key_drop_obsolete,
                        &sub_compact->compaction_job_stats);
      RecordCompactionIOStats();
      loop_cnt = 0;
    }

    sub_compact->compaction_job_stats.total_input_raw_key_bytes += key.size();
    sub_compact->compaction_job_stats.total_input_raw_value_bytes +=
        value.size();

    if (sub_compact->compaction->ShouldStopBefore(key) &&
        sub_compact->builder != nullptr) {
      status = FinishCompactionOutputFile(input->status(), sub_compact);
      if (!status.ok()) {
        break;
      }
    }

    if (ikey.type == kTypeDeletion) {
      sub_compact->compaction_job_stats.num_input_deletion_records++;
    }

    if (!has_current_user_key ||
        !cfd->user_comparator()->Equal(ikey.user_key,
                                       current_user_key.GetKey())) {
      // First occurrence of this user key
      current_user_key.SetKey(ikey.user_key);
      has_current_user_key = true;
      last_sequence_for_key = kMaxSequenceNumber;
      visible_in_snapshot = kMaxSequenceNumber;
      // apply the compaction filter to the first occurrence of the user key
      if (compaction_filter && ikey.type == kTypeValue &&
          (visible_at_tip_ || ikey.sequence > latest_snapshot_)) {
        // If the user has specified a compaction filter and the sequence
        // number is greater than any external snapshot, then invoke the
        // filter. If the return value of the compaction filter is true,
        // replace the entry with a deletion marker.
        bool value_changed = false;
        compaction_filter_value.clear();
        if (stats_ != nullptr) {
          timer.Start();
        }
        bool to_delete = compaction_filter->Filter(
            sub_compact->compaction->level(), ikey.user_key, value,
            &compaction_filter_value, &value_changed);
        total_filter_time += timer.ElapsedNanos();
        if (to_delete) {
          // make a copy of the original key and convert it to a delete
          delete_key.SetInternalKey(ExtractUserKey(key), ikey.sequence,
                                    kTypeDeletion);
          // anchor the key again
          key = delete_key.GetKey();
          // needed because ikey is backed by key
          ParseInternalKey(key, &ikey);
          // no value associated with delete
          value.clear();
          ++key_drop_user;
        } else if (value_changed) {
          value = compaction_filter_value;
        }
      }
    }

    // If there are no snapshots, then this kv affect visibility at tip.
    // Otherwise, search though all existing snapshots to find
    // the earlist snapshot that is affected by this kv.
    SequenceNumber prev_snapshot = 0;  // 0 means no previous snapshot
    SequenceNumber visible =
        visible_at_tip_ ? visible_at_tip_ : findEarliestVisibleSnapshot(
                                                ikey.sequence, &prev_snapshot);

    if (visible_in_snapshot == visible) {
      // If the earliest snapshot is which this key is visible in
      // is the same as the visibily of a previous instance of the
      // same key, then this kv is not visible in any snapshot.
      // Hidden by an newer entry for same user key
      // TODO: why not > ?
      assert(last_sequence_for_key >= ikey.sequence);
      ++key_drop_newer_entry;
      input->Next();  // (A)
    } else if (ikey.type == kTypeDeletion &&
               ikey.sequence <= earliest_snapshot_ &&
               sub_compact->compaction->KeyNotExistsBeyondOutputLevel(
                   ikey.user_key, &sub_compact->level_ptrs)) {
      // 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.
      ++key_drop_obsolete;
      input->Next();
    } else if (ikey.type == kTypeMerge) {
      if (!merge.HasOperator()) {
        LogToBuffer(log_buffer_, "Options::merge_operator is null.");
        status = Status::InvalidArgument(
            "merge_operator is not properly initialized.");
        break;
      }
      // We know the merge type entry is not hidden, otherwise we would
      // have hit (A)
      // We encapsulate the merge related state machine in a different
      // object to minimize change to the existing flow. Turn out this
      // logic could also be nicely re-used for memtable flush purge
      // optimization in BuildTable.
      merge.MergeUntil(input, prev_snapshot, bottommost_level_,
                       db_options_.statistics.get(), env_);

      // NOTE: key, value, and ikey refer to old entries.
      //       These will be correctly set below.
      const auto& keys = merge.keys();
      const auto& values = merge.values();
      assert(!keys.empty());
      assert(keys.size() == values.size());

      // We have a list of keys to write, write all keys in the list.
      for (auto key_iter = keys.rbegin(), value_iter = values.rbegin();
           !status.ok() || key_iter != keys.rend(); key_iter++, value_iter++) {
        key = Slice(*key_iter);
        value = Slice(*value_iter);
        bool valid_key __attribute__((__unused__)) =
            ParseInternalKey(key, &ikey);
        // MergeUntil stops when it encounters a corrupt key and does not
        // include them in the result, so we expect the keys here to valid.
        assert(valid_key);
        status = WriteKeyValue(key, value, ikey, input->status(), sub_compact);
      }
    } else {
      status = WriteKeyValue(key, value, ikey, input->status(), sub_compact);
      input->Next();
    }

    last_sequence_for_key = ikey.sequence;
    visible_in_snapshot = visible;
  }

  RecordTick(stats_, FILTER_OPERATION_TOTAL_TIME, total_filter_time);
  RecordDroppedKeys(&key_drop_user, &key_drop_newer_entry, &key_drop_obsolete,
                    &sub_compact->compaction_job_stats);
  RecordCompactionIOStats();

  if (status.ok() &&
      (shutting_down_->load(std::memory_order_acquire) || cfd->IsDropped())) {
    status = Status::ShutdownInProgress(
        "Database shutdown or Column family drop during compaction");
  }
  if (status.ok() && sub_compact->builder != nullptr) {
    status = FinishCompactionOutputFile(input->status(), sub_compact);
  }
  if (status.ok()) {
    status = input->status();
  }
  if (output_directory_ && !db_options_.disableDataSync) {
    // TODO(aekmekji): Maybe only call once after all subcompactions complete?
    output_directory_->Fsync();
  }

  if (measure_io_stats_) {
    sub_compact->compaction_job_stats.file_write_nanos +=
        iostats_context.write_nanos - prev_write_nanos;
    sub_compact->compaction_job_stats.file_fsync_nanos +=
        iostats_context.fsync_nanos - prev_fsync_nanos;
    sub_compact->compaction_job_stats.file_range_sync_nanos +=
        iostats_context.range_sync_nanos - prev_range_sync_nanos;
    sub_compact->compaction_job_stats.file_prepare_write_nanos +=
        iostats_context.prepare_write_nanos - prev_prepare_write_nanos;
    if (prev_perf_level != PerfLevel::kEnableTime) {
      SetPerfLevel(prev_perf_level);
    }
  }

  input_ptr.reset();
  sub_compact->status = status;
}

Status CompactionJob::WriteKeyValue(const Slice& key, const Slice& value,
    const ParsedInternalKey& ikey, const Status& input_status,
    SubCompactionState* sub_compact) {

  Slice newkey(key.data(), key.size());
  std::string kstr;

  // Zeroing out the sequence number leads to better compression.
  // If this is the bottommost level (no files in lower levels)
  // and the earliest snapshot is larger than this seqno
  // then we can squash the seqno to zero.
  if (bottommost_level_ && ikey.sequence < earliest_snapshot_ &&
      ikey.type != kTypeMerge) {
    assert(ikey.type != kTypeDeletion);
    // make a copy because updating in place would cause problems
    // with the priority queue that is managing the input key iterator
    kstr.assign(key.data(), key.size());
    UpdateInternalKey(&kstr, (uint64_t)0, ikey.type);
    newkey = Slice(kstr);
  }

  // Open output file if necessary
  if (sub_compact->builder == nullptr) {
    Status status = OpenCompactionOutputFile(sub_compact);
    if (!status.ok()) {
      return status;
    }
  }
  assert(sub_compact->builder != nullptr);
  assert(sub_compact->current_output() != nullptr);

  SequenceNumber seqno = GetInternalKeySeqno(newkey);
  if (sub_compact->builder->NumEntries() == 0) {
    sub_compact->current_output()->smallest.DecodeFrom(newkey);
    sub_compact->current_output()->smallest_seqno = seqno;
  } else {
    sub_compact->current_output()->smallest_seqno =
        std::min(sub_compact->current_output()->smallest_seqno, seqno);
  }
  sub_compact->current_output()->largest.DecodeFrom(newkey);
  sub_compact->builder->Add(newkey, value);
  sub_compact->num_output_records++;
  sub_compact->current_output()->largest_seqno =
    std::max(sub_compact->current_output()->largest_seqno, seqno);

  // Close output file if it is big enough
  Status status;
  if (sub_compact->builder->FileSize() >=
      sub_compact->compaction->max_output_file_size()) {
    status = FinishCompactionOutputFile(input_status, sub_compact);
  }

  return status;
}

void CompactionJob::RecordDroppedKeys(
    int64_t* key_drop_user,
    int64_t* key_drop_newer_entry,
    int64_t* key_drop_obsolete,
    CompactionJobStats* compaction_job_stats) {
  if (*key_drop_user > 0) {
    RecordTick(stats_, COMPACTION_KEY_DROP_USER, *key_drop_user);
    *key_drop_user = 0;
  }
  if (*key_drop_newer_entry > 0) {
    RecordTick(stats_, COMPACTION_KEY_DROP_NEWER_ENTRY, *key_drop_newer_entry);
    if (compaction_job_stats) {
      compaction_job_stats->num_records_replaced += *key_drop_newer_entry;
    }
    *key_drop_newer_entry = 0;
  }
  if (*key_drop_obsolete > 0) {
    RecordTick(stats_, COMPACTION_KEY_DROP_OBSOLETE, *key_drop_obsolete);
    if (compaction_job_stats) {
      compaction_job_stats->num_expired_deletion_records += *key_drop_obsolete;
    }
    *key_drop_obsolete = 0;
  }
}

Status CompactionJob::FinishCompactionOutputFile(const Status& input_status,
                                          SubCompactionState* sub_compact) {
  AutoThreadOperationStageUpdater stage_updater(
      ThreadStatus::STAGE_COMPACTION_SYNC_FILE);
  assert(sub_compact != nullptr);
  assert(sub_compact->outfile);
  assert(sub_compact->builder != nullptr);
  assert(sub_compact->current_output() != nullptr);

  const uint64_t output_number = sub_compact->current_output()->number;
  const uint32_t output_path_id = sub_compact->current_output()->path_id;
  assert(output_number != 0);

  TableProperties table_properties;
  // Check for iterator errors
  Status s = input_status;
  const uint64_t current_entries = sub_compact->builder->NumEntries();
  sub_compact->current_output()->need_compaction =
      sub_compact->builder->NeedCompact();
  if (s.ok()) {
    s = sub_compact->builder->Finish();
  } else {
    sub_compact->builder->Abandon();
  }
  const uint64_t current_bytes = sub_compact->builder->FileSize();
  sub_compact->current_output()->file_size = current_bytes;
  sub_compact->total_bytes += current_bytes;

  // Finish and check for file errors
  if (s.ok() && !db_options_.disableDataSync) {
    StopWatch sw(env_, stats_, COMPACTION_OUTFILE_SYNC_MICROS);
    s = sub_compact->outfile->Sync(db_options_.use_fsync);
  }
  if (s.ok()) {
    s = sub_compact->outfile->Close();
  }
  sub_compact->outfile.reset();

  if (s.ok() && current_entries > 0) {
    // Verify that the table is usable
    ColumnFamilyData* cfd = sub_compact->compaction->column_family_data();
    FileDescriptor fd(output_number, output_path_id, current_bytes);
    Iterator* iter = cfd->table_cache()->NewIterator(
        ReadOptions(), env_options_, cfd->internal_comparator(), fd, nullptr,
        cfd->internal_stats()->GetFileReadHist(
            compact_->compaction->output_level()),
        false);
    s = iter->status();

    if (s.ok() && paranoid_file_checks_) {
      for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {}
      s = iter->status();
    }

    delete iter;
    if (s.ok()) {
      TableFileCreationInfo info(sub_compact->builder->GetTableProperties());
      info.db_name = dbname_;
      info.cf_name = cfd->GetName();
      info.file_path = TableFileName(cfd->ioptions()->db_paths,
                                     fd.GetNumber(), fd.GetPathId());
      info.file_size = fd.GetFileSize();
      info.job_id = job_id_;
      Log(InfoLogLevel::INFO_LEVEL, db_options_.info_log,
          "[%s] [JOB %d] Generated table #%" PRIu64 ": %" PRIu64
          " keys, %" PRIu64 " bytes%s",
          cfd->GetName().c_str(), job_id_, output_number, current_entries,
          current_bytes,
          sub_compact->current_output()->need_compaction ? " (need compaction)"
                                                      : "");
      EventHelpers::LogAndNotifyTableFileCreation(
          event_logger_, cfd->ioptions()->listeners, fd, info);
    }
  }
  sub_compact->builder.reset();
  return s;
}

Status CompactionJob::InstallCompactionResults(
    const MutableCFOptions& mutable_cf_options, InstrumentedMutex* db_mutex) {
  db_mutex->AssertHeld();

  auto* compaction = compact_->compaction;
  // paranoia: verify that the files that we started with
  // still exist in the current version and in the same original level.
  // This ensures that a concurrent compaction did not erroneously
  // pick the same files to compact_.
  if (!versions_->VerifyCompactionFileConsistency(compaction)) {
    Compaction::InputLevelSummaryBuffer inputs_summary;

    Log(InfoLogLevel::ERROR_LEVEL, db_options_.info_log,
        "[%s] [JOB %d] Compaction %s aborted",
        compaction->column_family_data()->GetName().c_str(), job_id_,
        compaction->InputLevelSummary(&inputs_summary));
    return Status::Corruption("Compaction input files inconsistent");
  }

  {
    Compaction::InputLevelSummaryBuffer inputs_summary;
    Log(InfoLogLevel::INFO_LEVEL, db_options_.info_log,
        "[%s] [JOB %d] Compacted %s => %" PRIu64 " bytes",
        compaction->column_family_data()->GetName().c_str(), job_id_,
        compaction->InputLevelSummary(&inputs_summary), compact_->total_bytes);
  }

  // Add compaction outputs
  compaction->AddInputDeletions(compact_->compaction->edit());

  for (SubCompactionState& sub_compact : compact_->sub_compact_states) {
    for (size_t i = 0; i < sub_compact.outputs.size(); i++) {
      const SubCompactionState::Output& out = sub_compact.outputs[i];
      compaction->edit()->AddFile(compaction->output_level(), out.number,
                                  out.path_id, out.file_size, out.smallest,
                                  out.largest, out.smallest_seqno,
                                  out.largest_seqno, out.need_compaction);
    }
  }
  return versions_->LogAndApply(compaction->column_family_data(),
                                mutable_cf_options, compaction->edit(),
                                db_mutex, db_directory_);
}

// Given a sequence number, return the sequence number of the
// earliest snapshot that this sequence number is visible in.
// The snapshots themselves are arranged in ascending order of
// sequence numbers.
// Employ a sequential search because the total number of
// snapshots are typically small.
inline SequenceNumber CompactionJob::findEarliestVisibleSnapshot(
    SequenceNumber in, SequenceNumber* prev_snapshot) {
  assert(existing_snapshots_.size());
  SequenceNumber prev __attribute__((unused)) = 0;
  for (const auto cur : existing_snapshots_) {
    assert(prev <= cur);
    if (cur >= in) {
      *prev_snapshot = prev;
      return cur;
    }
    prev = cur;  // assignment
    assert(prev);
  }
  Log(InfoLogLevel::WARN_LEVEL, db_options_.info_log,
      "CompactionJob is not able to find snapshot"
      " with SeqId later than %" PRIu64
      ": current MaxSeqId is %" PRIu64 "",
      in, existing_snapshots_[existing_snapshots_.size() - 1]);
  assert(0);
  return 0;
}

void CompactionJob::RecordCompactionIOStats() {
  RecordTick(stats_, COMPACT_READ_BYTES, IOSTATS(bytes_read));
  ThreadStatusUtil::IncreaseThreadOperationProperty(
      ThreadStatus::COMPACTION_BYTES_READ, IOSTATS(bytes_read));
  IOSTATS_RESET(bytes_read);
  RecordTick(stats_, COMPACT_WRITE_BYTES, IOSTATS(bytes_written));
  ThreadStatusUtil::IncreaseThreadOperationProperty(
      ThreadStatus::COMPACTION_BYTES_WRITTEN, IOSTATS(bytes_written));
  IOSTATS_RESET(bytes_written);
}

Status CompactionJob::OpenCompactionOutputFile(SubCompactionState*
                                                                sub_compact) {
  assert(sub_compact != nullptr);
  assert(sub_compact->builder == nullptr);
  // no need to lock because VersionSet::next_file_number_ is atomic
  uint64_t file_number = versions_->NewFileNumber();
  // Make the output file
  unique_ptr<WritableFile> writable_file;
  std::string fname = TableFileName(db_options_.db_paths, file_number,
                                    sub_compact->compaction->output_path_id());
  Status s = env_->NewWritableFile(fname, &writable_file, env_options_);
  if (!s.ok()) {
    Log(InfoLogLevel::ERROR_LEVEL, db_options_.info_log,
        "[%s] [JOB %d] OpenCompactionOutputFiles for table #%" PRIu64
        " fails at NewWritableFile with status %s",
        sub_compact->compaction->column_family_data()->GetName().c_str(),
        job_id_, file_number, s.ToString().c_str());
    LogFlush(db_options_.info_log);
    return s;
  }
  SubCompactionState::Output out;
  out.number = file_number;
  out.path_id = sub_compact->compaction->output_path_id();
  out.smallest.Clear();
  out.largest.Clear();
  out.smallest_seqno = out.largest_seqno = 0;

  sub_compact->outputs.push_back(out);
  writable_file->SetIOPriority(Env::IO_LOW);
  writable_file->SetPreallocationBlockSize(static_cast<size_t>(
      sub_compact->compaction->OutputFilePreallocationSize()));
  sub_compact->outfile.reset(
      new WritableFileWriter(std::move(writable_file), env_options_));

  ColumnFamilyData* cfd = sub_compact->compaction->column_family_data();
  bool skip_filters = false;

  // If the Column family flag is to only optimize filters for hits,
  // we can skip creating filters if this is the bottommost_level where
  // data is going to be found
  //
  if (cfd->ioptions()->optimize_filters_for_hits && bottommost_level_) {
    skip_filters = true;
  }

  sub_compact->builder.reset(NewTableBuilder(
      *cfd->ioptions(), cfd->internal_comparator(),
      cfd->int_tbl_prop_collector_factories(), sub_compact->outfile.get(),
      sub_compact->compaction->output_compression(),
      cfd->ioptions()->compression_opts, skip_filters));
  LogFlush(db_options_.info_log);
  return s;
}

void CompactionJob::CleanupCompaction() {
  for (SubCompactionState& sub_compact : compact_->sub_compact_states) {
    const auto& sub_status = sub_compact.status;

    if (sub_compact.builder != nullptr) {
      // May happen if we get a shutdown call in the middle of compaction
      sub_compact.builder->Abandon();
      sub_compact.builder.reset();
    } else {
      assert(!sub_status.ok() || sub_compact.outfile == nullptr);
    }
    for (size_t i = 0; i < sub_compact.outputs.size(); i++) {
      const SubCompactionState::Output& out = sub_compact.outputs[i];

      // If this file was inserted into the table cache then remove
      // them here because this compaction was not committed.
      if (!sub_status.ok()) {
        TableCache::Evict(table_cache_.get(), out.number);
      }
    }
  }
  delete compact_;
  compact_ = nullptr;
}

#ifndef ROCKSDB_LITE
namespace {
void CopyPrefix(
    const Slice& src, size_t prefix_length, std::string* dst) {
  assert(prefix_length > 0);
  size_t length = src.size() > prefix_length ? prefix_length : src.size();
  dst->assign(src.data(), length);
}
}  // namespace

#endif  // !ROCKSDB_LITE

void CompactionJob::UpdateCompactionStats() {
  Compaction* compaction = compact_->compaction;
  compaction_stats_.num_input_files_in_non_output_levels = 0;
  compaction_stats_.num_input_files_in_output_level = 0;
  for (int input_level = 0;
       input_level < static_cast<int>(compaction->num_input_levels());
       ++input_level) {
    if (compaction->start_level() + input_level
        != compaction->output_level()) {
      UpdateCompactionInputStatsHelper(
          &compaction_stats_.num_input_files_in_non_output_levels,
          &compaction_stats_.bytes_read_non_output_levels,
          input_level);
    } else {
      UpdateCompactionInputStatsHelper(
          &compaction_stats_.num_input_files_in_output_level,
          &compaction_stats_.bytes_read_output_level,
          input_level);
    }
  }

  for (const auto& sub_compact : compact_->sub_compact_states) {
    size_t num_output_files = sub_compact.outputs.size();
    if (sub_compact.builder != nullptr) {
      // An error occurred so ignore the last output.
      assert(num_output_files > 0);
      --num_output_files;
    }
    compaction_stats_.num_output_files += static_cast<int>(num_output_files);

    for (size_t i = 0; i < num_output_files; i++) {
      compaction_stats_.bytes_written += sub_compact.outputs[i].file_size;
    }
    if (sub_compact.num_input_records > sub_compact.num_output_records) {
      compaction_stats_.num_dropped_records +=
          sub_compact.num_input_records - sub_compact.num_output_records;
    }
  }
}

void CompactionJob::UpdateCompactionInputStatsHelper(
    int* num_files, uint64_t* bytes_read, int input_level) {
  const Compaction* compaction = compact_->compaction;
  auto num_input_files = compaction->num_input_files(input_level);
  *num_files += static_cast<int>(num_input_files);

  for (size_t i = 0; i < num_input_files; ++i) {
    const auto* file_meta = compaction->input(input_level, i);
    *bytes_read += file_meta->fd.GetFileSize();
    compaction_stats_.num_input_records +=
        static_cast<uint64_t>(file_meta->num_entries);
  }
}

void CompactionJob::UpdateCompactionJobStats(
    const InternalStats::CompactionStats& stats) const {
#ifndef ROCKSDB_LITE
  if (compaction_job_stats_) {
    compaction_job_stats_->elapsed_micros = stats.micros;

    // input information
    compaction_job_stats_->total_input_bytes =
        stats.bytes_read_non_output_levels +
        stats.bytes_read_output_level;
    compaction_job_stats_->num_input_records =
        compact_->num_input_records;
    compaction_job_stats_->num_input_files =
        stats.num_input_files_in_non_output_levels +
        stats.num_input_files_in_output_level;
    compaction_job_stats_->num_input_files_at_output_level =
        stats.num_input_files_in_output_level;

    // output information
    compaction_job_stats_->total_output_bytes = stats.bytes_written;
    compaction_job_stats_->num_output_records =
        compact_->num_output_records;
    compaction_job_stats_->num_output_files = stats.num_output_files;

    if (compact_->NumOutputFiles() > 0U) {
      CopyPrefix(
          compact_->SmallestUserKey(),
          CompactionJobStats::kMaxPrefixLength,
          &compaction_job_stats_->smallest_output_key_prefix);
      CopyPrefix(
          compact_->LargestUserKey(),
          CompactionJobStats::kMaxPrefixLength,
          &compaction_job_stats_->largest_output_key_prefix);
    }
  }
#endif  // !ROCKSDB_LITE
}

void CompactionJob::LogCompaction() {
  Compaction* compaction = compact_->compaction;
  ColumnFamilyData* cfd = compaction->column_family_data();

  // Let's check if anything will get logged. Don't prepare all the info if
  // we're not logging
  if (db_options_.info_log_level <= InfoLogLevel::INFO_LEVEL) {
    Compaction::InputLevelSummaryBuffer inputs_summary;
    Log(InfoLogLevel::INFO_LEVEL, db_options_.info_log,
        "[%s] [JOB %d] Compacting %s, score %.2f", cfd->GetName().c_str(),
        job_id_, compaction->InputLevelSummary(&inputs_summary),
        compaction->score());
    char scratch[2345];
    compaction->Summary(scratch, sizeof(scratch));
    Log(InfoLogLevel::INFO_LEVEL, db_options_.info_log,
        "[%s] Compaction start summary: %s\n", cfd->GetName().c_str(), scratch);
    // build event logger report
    auto stream = event_logger_->Log();
    stream << "job" << job_id_ << "event"
           << "compaction_started";
    for (size_t i = 0; i < compaction->num_input_levels(); ++i) {
      stream << ("files_L" + ToString(compaction->level(i)));
      stream.StartArray();
      for (auto f : *compaction->inputs(i)) {
        stream << f->fd.GetNumber();
      }
      stream.EndArray();
    }
    stream << "score" << compaction->score() << "input_data_size"
           << compaction->CalculateTotalInputSize();
  }
}

}  // namespace rocksdb