3bfd3d39a3
Summary: Currently, transactions can fail even if there is no actual write conflict. This is due to relying on only the memtables to check for write-conflicts. Users have to tune memtable settings to try to avoid this, but it's hard to figure out exactly how to tune these settings. With this diff, TransactionDB will use both memtables and SST files to determine if there are any write conflicts. This relies on the fact that BlockBasedTable stores sequence numbers for all writes that happen after any open snapshot. Also, D50295 is needed to prevent SingleDelete from disappearing writes (the TODOs in this test code will be fixed once the other diff is approved and merged). Note that Optimistic transactions will still rely on tuning memtable settings as we do not want to read from SST while on the write thread. Also, memtable settings can still be used to reduce how often TransactionDB needs to read SST files. Test Plan: unit tests, db bench Reviewers: rven, yhchiang, kradhakrishnan, IslamAbdelRahman, sdong Reviewed By: sdong Subscribers: dhruba, leveldb, yoshinorim Differential Revision: https://reviews.facebook.net/D50475
439 lines
18 KiB
C++
439 lines
18 KiB
C++
// 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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// Copyright (c) 2013, Facebook, Inc. All rights reserved.
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// This source code is licensed under the BSD-style license found in the
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// LICENSE file in the root directory of this source tree. An additional grant
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// of patent rights can be found in the PATENTS file in the same directory.
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#include "db/compaction_iterator.h"
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#include "table/internal_iterator.h"
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namespace rocksdb {
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CompactionIterator::CompactionIterator(
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InternalIterator* input, const Comparator* cmp, MergeHelper* merge_helper,
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SequenceNumber last_sequence, std::vector<SequenceNumber>* snapshots,
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SequenceNumber earliest_write_conflict_snapshot, Env* env,
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bool expect_valid_internal_key, Compaction* compaction,
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const CompactionFilter* compaction_filter, LogBuffer* log_buffer)
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: input_(input),
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cmp_(cmp),
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merge_helper_(merge_helper),
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snapshots_(snapshots),
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earliest_write_conflict_snapshot_(earliest_write_conflict_snapshot),
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env_(env),
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expect_valid_internal_key_(expect_valid_internal_key),
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compaction_(compaction),
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compaction_filter_(compaction_filter),
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log_buffer_(log_buffer),
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merge_out_iter_(merge_helper_) {
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assert(compaction_filter_ == nullptr || compaction_ != nullptr);
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bottommost_level_ =
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compaction_ == nullptr ? false : compaction_->bottommost_level();
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if (compaction_ != nullptr) {
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level_ptrs_ = std::vector<size_t>(compaction_->number_levels(), 0);
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}
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if (snapshots_->size() == 0) {
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// optimize for fast path if there are no snapshots
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visible_at_tip_ = last_sequence;
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earliest_snapshot_ = visible_at_tip_;
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latest_snapshot_ = 0;
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} else {
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visible_at_tip_ = 0;
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earliest_snapshot_ = snapshots_->at(0);
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latest_snapshot_ = snapshots_->back();
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}
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if (compaction_filter_ != nullptr && compaction_filter_->IgnoreSnapshots()) {
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ignore_snapshots_ = true;
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} else {
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ignore_snapshots_ = false;
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}
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}
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void CompactionIterator::ResetRecordCounts() {
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iter_stats_.num_record_drop_user = 0;
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iter_stats_.num_record_drop_hidden = 0;
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iter_stats_.num_record_drop_obsolete = 0;
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}
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void CompactionIterator::SeekToFirst() {
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NextFromInput();
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PrepareOutput();
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}
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void CompactionIterator::Next() {
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// If there is a merge output, return it before continuing to process the
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// input.
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if (merge_out_iter_.Valid()) {
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merge_out_iter_.Next();
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// Check if we returned all records of the merge output.
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if (merge_out_iter_.Valid()) {
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key_ = merge_out_iter_.key();
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value_ = merge_out_iter_.value();
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bool valid_key __attribute__((__unused__)) =
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ParseInternalKey(key_, &ikey_);
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// MergeUntil stops when it encounters a corrupt key and does not
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// include them in the result, so we expect the keys here to be valid.
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assert(valid_key);
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// Keep current_key_ in sync.
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current_key_.UpdateInternalKey(ikey_.sequence, ikey_.type);
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key_ = current_key_.GetKey();
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ikey_.user_key = current_key_.GetUserKey();
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valid_ = true;
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} else {
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// MergeHelper moves the iterator to the first record after the merged
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// records, so even though we reached the end of the merge output, we do
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// not want to advance the iterator.
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NextFromInput();
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}
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} else {
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// Only advance the input iterator if there is no merge output and the
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// iterator is not already at the next record.
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if (!at_next_) {
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input_->Next();
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}
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NextFromInput();
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}
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if (valid_) {
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// Record that we've ouputted a record for the current key.
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has_outputted_key_ = true;
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}
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PrepareOutput();
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}
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void CompactionIterator::NextFromInput() {
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at_next_ = false;
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valid_ = false;
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while (!valid_ && input_->Valid()) {
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key_ = input_->key();
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value_ = input_->value();
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iter_stats_.num_input_records++;
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if (!ParseInternalKey(key_, &ikey_)) {
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// If `expect_valid_internal_key_` is false, return the corrupted key
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// and let the caller decide what to do with it.
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// TODO(noetzli): We should have a more elegant solution for this.
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if (expect_valid_internal_key_) {
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assert(!"Corrupted internal key not expected.");
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status_ = Status::Corruption("Corrupted internal key not expected.");
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break;
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}
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key_ = current_key_.SetKey(key_);
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has_current_user_key_ = false;
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current_user_key_sequence_ = kMaxSequenceNumber;
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current_user_key_snapshot_ = 0;
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iter_stats_.num_input_corrupt_records++;
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valid_ = true;
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break;
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}
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// Update input statistics
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if (ikey_.type == kTypeDeletion || ikey_.type == kTypeSingleDeletion) {
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iter_stats_.num_input_deletion_records++;
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}
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iter_stats_.total_input_raw_key_bytes += key_.size();
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iter_stats_.total_input_raw_value_bytes += value_.size();
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// Check whether the user key changed. After this if statement current_key_
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// is a copy of the current input key (maybe converted to a delete by the
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// compaction filter). ikey_.user_key is pointing to the copy.
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if (!has_current_user_key_ ||
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!cmp_->Equal(ikey_.user_key, current_user_key_)) {
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// First occurrence of this user key
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key_ = current_key_.SetKey(key_, &ikey_);
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current_user_key_ = ikey_.user_key;
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has_current_user_key_ = true;
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has_outputted_key_ = false;
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current_user_key_sequence_ = kMaxSequenceNumber;
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current_user_key_snapshot_ = 0;
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// apply the compaction filter to the first occurrence of the user key
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if (compaction_filter_ != nullptr && ikey_.type == kTypeValue &&
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(visible_at_tip_ || ikey_.sequence > latest_snapshot_ ||
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ignore_snapshots_)) {
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// If the user has specified a compaction filter and the sequence
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// number is greater than any external snapshot, then invoke the
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// filter. If the return value of the compaction filter is true,
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// replace the entry with a deletion marker.
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bool value_changed = false;
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bool to_delete = false;
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compaction_filter_value_.clear();
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{
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StopWatchNano timer(env_, true);
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to_delete = compaction_filter_->Filter(
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compaction_->level(), ikey_.user_key, value_,
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&compaction_filter_value_, &value_changed);
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iter_stats_.total_filter_time +=
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env_ != nullptr ? timer.ElapsedNanos() : 0;
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}
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if (to_delete) {
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// convert the current key to a delete
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ikey_.type = kTypeDeletion;
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current_key_.UpdateInternalKey(ikey_.sequence, kTypeDeletion);
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// no value associated with delete
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value_.clear();
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iter_stats_.num_record_drop_user++;
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} else if (value_changed) {
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value_ = compaction_filter_value_;
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}
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}
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} else {
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// Update the current key to reflect the new sequence number/type without
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// copying the user key.
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// TODO(rven): Compaction filter does not process keys in this path
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// Need to have the compaction filter process multiple versions
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// if we have versions on both sides of a snapshot
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current_key_.UpdateInternalKey(ikey_.sequence, ikey_.type);
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key_ = current_key_.GetKey();
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ikey_.user_key = current_key_.GetUserKey();
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}
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// If there are no snapshots, then this kv affect visibility at tip.
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// Otherwise, search though all existing snapshots to find the earliest
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// snapshot that is affected by this kv.
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SequenceNumber last_sequence __attribute__((__unused__)) =
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current_user_key_sequence_;
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current_user_key_sequence_ = ikey_.sequence;
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SequenceNumber last_snapshot = current_user_key_snapshot_;
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SequenceNumber prev_snapshot = 0; // 0 means no previous snapshot
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current_user_key_snapshot_ =
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visible_at_tip_ ? visible_at_tip_ : findEarliestVisibleSnapshot(
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ikey_.sequence, &prev_snapshot);
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if (clear_and_output_next_key_) {
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// In the previous iteration we encountered a single delete that we could
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// not compact out. We will keep this Put, but can drop it's data.
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// (See Optimization 3, below.)
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assert(ikey_.type == kTypeValue);
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assert(current_user_key_snapshot_ == last_snapshot);
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value_.clear();
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valid_ = true;
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clear_and_output_next_key_ = false;
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} else if (ikey_.type == kTypeSingleDeletion) {
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// We can compact out a SingleDelete if:
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// 1) We encounter the corresponding PUT -OR- we know that this key
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// doesn't appear past this output level
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// =AND=
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// 2) We've already returned a record in this snapshot -OR-
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// there are no earlier earliest_write_conflict_snapshot.
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//
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// Rule 1 is needed for SingleDelete correctness. Rule 2 is needed to
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// allow Transactions to do write-conflict checking (if we compacted away
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// all keys, then we wouldn't know that a write happened in this
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// snapshot). If there is no earlier snapshot, then we know that there
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// are no active transactions that need to know about any writes.
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//
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// Optimization 3:
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// If we encounter a SingleDelete followed by a PUT and Rule 2 is NOT
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// true, then we must output a SingleDelete. In this case, we will decide
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// to also output the PUT. While we are compacting less by outputting the
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// PUT now, hopefully this will lead to better compaction in the future
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// when Rule 2 is later true (Ie, We are hoping we can later compact out
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// both the SingleDelete and the Put, while we couldn't if we only
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// outputted the SingleDelete now).
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// In this case, we can save space by removing the PUT's value as it will
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// never be read.
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//
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// Deletes and Merges are not supported on the same key that has a
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// SingleDelete as it is not possible to correctly do any partial
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// compaction of such a combination of operations. The result of mixing
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// those operations for a given key is documented as being undefined. So
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// we can choose how to handle such a combinations of operations. We will
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// try to compact out as much as we can in these cases.
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// The easiest way to process a SingleDelete during iteration is to peek
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// ahead at the next key.
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ParsedInternalKey next_ikey;
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input_->Next();
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// Check whether the next key exists, is not corrupt, and is the same key
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// as the single delete.
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if (input_->Valid() && ParseInternalKey(input_->key(), &next_ikey) &&
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cmp_->Equal(ikey_.user_key, next_ikey.user_key)) {
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// Check whether the next key belongs to the same snapshot as the
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// SingleDelete.
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if (prev_snapshot == 0 || next_ikey.sequence > prev_snapshot) {
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if (next_ikey.type == kTypeSingleDeletion) {
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// We encountered two SingleDeletes in a row. This could be due to
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// unexpected user input.
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// Skip the first SingleDelete and let the next iteration decide how
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// to handle the second SingleDelete
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// First SingleDelete has been skipped since we already called
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// input_->Next().
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++iter_stats_.num_record_drop_obsolete;
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} else if ((ikey_.sequence <= earliest_write_conflict_snapshot_) ||
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has_outputted_key_) {
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// Found a matching value, we can drop the single delete and the
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// value. It is safe to drop both records since we've already
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// outputted a key in this snapshot, or there is no earlier
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// snapshot (Rule 2 above).
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// Note: it doesn't matter whether the second key is a Put or if it
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// is an unexpected Merge or Delete. We will compact it out
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// either way.
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++iter_stats_.num_record_drop_hidden;
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++iter_stats_.num_record_drop_obsolete;
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// Already called input_->Next() once. Call it a second time to
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// skip past the second key.
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input_->Next();
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} else {
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// Found a matching value, but we cannot drop both keys since
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// there is an earlier snapshot and we need to leave behind a record
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// to know that a write happened in this snapshot (Rule 2 above).
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// Clear the value and output the SingleDelete. (The value will be
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// outputted on the next iteration.)
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++iter_stats_.num_record_drop_hidden;
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// Setting valid_ to true will output the current SingleDelete
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valid_ = true;
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// Set up the Put to be outputted in the next iteration.
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// (Optimization 3).
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clear_and_output_next_key_ = true;
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}
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} else {
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// We hit the next snapshot without hitting a put, so the iterator
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// returns the single delete.
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valid_ = true;
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}
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} else {
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// We are at the end of the input, could not parse the next key, or hit
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// the next key. The iterator returns the single delete if the key
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// possibly exists beyond the current output level. We set
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// has_current_user_key to false so that if the iterator is at the next
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// key, we do not compare it again against the previous key at the next
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// iteration. If the next key is corrupt, we return before the
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// comparison, so the value of has_current_user_key does not matter.
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has_current_user_key_ = false;
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if (compaction_ != nullptr && ikey_.sequence <= earliest_snapshot_ &&
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compaction_->KeyNotExistsBeyondOutputLevel(ikey_.user_key,
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&level_ptrs_)) {
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// Key doesn't exist outside of this range.
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// Can compact out this SingleDelete.
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++iter_stats_.num_record_drop_obsolete;
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} else {
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// Output SingleDelete
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valid_ = true;
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}
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}
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if (valid_) {
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at_next_ = true;
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}
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} else if (last_snapshot == current_user_key_snapshot_) {
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// If the earliest snapshot is which this key is visible in
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// is the same as the visibility of a previous instance of the
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// same key, then this kv is not visible in any snapshot.
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// Hidden by an newer entry for same user key
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// TODO: why not > ?
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//
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// Note: Dropping this key will not affect TransactionDB write-conflict
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// checking since there has already been a record returned for this key
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// in this snapshot.
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assert(last_sequence >= current_user_key_sequence_);
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++iter_stats_.num_record_drop_hidden; // (A)
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input_->Next();
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} else if (compaction_ != nullptr && ikey_.type == kTypeDeletion &&
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ikey_.sequence <= earliest_snapshot_ &&
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compaction_->KeyNotExistsBeyondOutputLevel(ikey_.user_key,
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&level_ptrs_)) {
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// TODO(noetzli): This is the only place where we use compaction_
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// (besides the constructor). We should probably get rid of this
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// dependency and find a way to do similar filtering during flushes.
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//
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// For this user key:
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// (1) there is no data in higher levels
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// (2) data in lower levels will have larger sequence numbers
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// (3) data in layers that are being compacted here and have
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// smaller sequence numbers will be dropped in the next
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// few iterations of this loop (by rule (A) above).
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// Therefore this deletion marker is obsolete and can be dropped.
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//
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// Note: Dropping this Delete will not affect TransactionDB
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// write-conflict checking since it is earlier than any snapshot.
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++iter_stats_.num_record_drop_obsolete;
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input_->Next();
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} else if (ikey_.type == kTypeMerge) {
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if (!merge_helper_->HasOperator()) {
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LogToBuffer(log_buffer_, "Options::merge_operator is null.");
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status_ = Status::InvalidArgument(
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"merge_operator is not properly initialized.");
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return;
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}
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// We know the merge type entry is not hidden, otherwise we would
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// have hit (A)
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// We encapsulate the merge related state machine in a different
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// object to minimize change to the existing flow.
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merge_helper_->MergeUntil(input_, prev_snapshot, bottommost_level_);
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merge_out_iter_.SeekToFirst();
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if (merge_out_iter_.Valid()) {
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// NOTE: key, value, and ikey_ refer to old entries.
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// These will be correctly set below.
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key_ = merge_out_iter_.key();
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value_ = merge_out_iter_.value();
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bool valid_key __attribute__((__unused__)) =
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ParseInternalKey(key_, &ikey_);
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// MergeUntil stops when it encounters a corrupt key and does not
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// include them in the result, so we expect the keys here to valid.
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assert(valid_key);
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// Keep current_key_ in sync.
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current_key_.UpdateInternalKey(ikey_.sequence, ikey_.type);
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key_ = current_key_.GetKey();
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ikey_.user_key = current_key_.GetUserKey();
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valid_ = true;
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} else {
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// all merge operands were filtered out. reset the user key, since the
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// batch consumed by the merge operator should not shadow any keys
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// coming after the merges
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has_current_user_key_ = false;
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}
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} else {
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valid_ = true;
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}
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}
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}
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void CompactionIterator::PrepareOutput() {
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// Zeroing out the sequence number leads to better compression.
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// If this is the bottommost level (no files in lower levels)
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// and the earliest snapshot is larger than this seqno
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// then we can squash the seqno to zero.
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// This is safe for TransactionDB write-conflict checking since transactions
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// only care about sequence number larger than any active snapshots.
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if (bottommost_level_ && valid_ && ikey_.sequence < earliest_snapshot_ &&
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ikey_.type != kTypeMerge) {
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assert(ikey_.type != kTypeDeletion && ikey_.type != kTypeSingleDeletion);
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ikey_.sequence = 0;
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current_key_.UpdateInternalKey(0, ikey_.type);
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}
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}
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inline SequenceNumber CompactionIterator::findEarliestVisibleSnapshot(
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SequenceNumber in, SequenceNumber* prev_snapshot) {
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assert(snapshots_->size());
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SequenceNumber prev __attribute__((unused)) = 0;
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for (const auto cur : *snapshots_) {
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assert(prev <= cur);
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if (cur >= in) {
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*prev_snapshot = prev;
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return cur;
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}
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prev = cur;
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assert(prev);
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}
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*prev_snapshot = prev;
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return kMaxSequenceNumber;
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}
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} // namespace rocksdb
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