Introduce FullMergeV2 (eliminate memcpy from merge operators)

Summary: This diff update the code to pin the merge operator operands while the merge operation is done, so that we can eliminate the memcpy cost, to do that we need a new public API for FullMerge that replace the std::deque<std::string> with std::vector<Slice> This diff is stacked on top of D56493 and D56511 In this diff we - Update FullMergeV2 arguments to be encapsulated in MergeOperationInput and MergeOperationOutput which will make it easier to add new arguments in the future - Replace std::deque<std::string> with std::vector<Slice> to pass operands - Replace MergeContext std::deque with std::vector (based on a simple benchmark I ran https://gist.github.com/IslamAbdelRahman/78fc86c9ab9f52b1df791e58943fb187) - Allow FullMergeV2 output to be an existing operand ``` [Everything in Memtable | 10K operands | 10 KB each | 1 operand per key] DEBUG_LEVEL=0 make db_bench -j64 && ./db_bench --benchmarks="mergerandom,readseq,readseq,readseq,readseq,readseq" --merge_operator="max" --merge_keys=10000 --num=10000 --disable_auto_compactions --value_size=10240 --write_buffer_size=1000000000 [FullMergeV2] readseq : 0.607 micros/op 1648235 ops/sec; 16121.2 MB/s readseq : 0.478 micros/op 2091546 ops/sec; 20457.2 MB/s readseq : 0.252 micros/op 3972081 ops/sec; 38850.5 MB/s readseq : 0.237 micros/op 4218328 ops/sec; 41259.0 MB/s readseq : 0.247 micros/op 4043927 ops/sec; 39553.2 MB/s [master] readseq : 3.935 micros/op 254140 ops/sec; 2485.7 MB/s readseq : 3.722 micros/op 268657 ops/sec; 2627.7 MB/s readseq : 3.149 micros/op 317605 ops/sec; 3106.5 MB/s readseq : 3.125 micros/op 320024 ops/sec; 3130.1 MB/s readseq : 4.075 micros/op 245374 ops/sec; 2400.0 MB/s ``` ``` [Everything in Memtable | 10K operands | 10 KB each | 10 operand per key] DEBUG_LEVEL=0 make db_bench -j64 && ./db_bench --benchmarks="mergerandom,readseq,readseq,readseq,readseq,readseq" --merge_operator="max" --merge_keys=1000 --num=10000 --disable_auto_compactions --value_size=10240 --write_buffer_size=1000000000 [FullMergeV2] readseq : 3.472 micros/op 288018 ops/sec; 2817.1 MB/s readseq : 2.304 micros/op 434027 ops/sec; 4245.2 MB/s readseq : 1.163 micros/op 859845 ops/sec; 8410.0 MB/s readseq : 1.192 micros/op 838926 ops/sec; 8205.4 MB/s readseq : 1.250 micros/op 800000 ops/sec; 7824.7 MB/s [master] readseq : 24.025 micros/op 41623 ops/sec; 407.1 MB/s readseq : 18.489 micros/op 54086 ops/sec; 529.0 MB/s readseq : 18.693 micros/op 53495 ops/sec; 523.2 MB/s readseq : 23.621 micros/op 42335 ops/sec; 414.1 MB/s readseq : 18.775 micros/op 53262 ops/sec; 521.0 MB/s ``` ``` [Everything in Block cache | 10K operands | 10 KB each | 1 operand per key] [FullMergeV2] $ DEBUG_LEVEL=0 make db_bench -j64 && ./db_bench --benchmarks="readseq,readseq,readseq,readseq,readseq" --merge_operator="max" --num=100000 --db="/dev/shm/merge-random-10K-10KB" --cache_size=1000000000 --use_existing_db --disable_auto_compactions readseq : 14.741 micros/op 67837 ops/sec; 663.5 MB/s readseq : 1.029 micros/op 971446 ops/sec; 9501.6 MB/s readseq : 0.974 micros/op 1026229 ops/sec; 10037.4 MB/s readseq : 0.965 micros/op 1036080 ops/sec; 10133.8 MB/s readseq : 0.943 micros/op 1060657 ops/sec; 10374.2 MB/s [master] readseq : 16.735 micros/op 59755 ops/sec; 584.5 MB/s readseq : 3.029 micros/op 330151 ops/sec; 3229.2 MB/s readseq : 3.136 micros/op 318883 ops/sec; 3119.0 MB/s readseq : 3.065 micros/op 326245 ops/sec; 3191.0 MB/s readseq : 3.014 micros/op 331813 ops/sec; 3245.4 MB/s ``` ``` [Everything in Block cache | 10K operands | 10 KB each | 10 operand per key] DEBUG_LEVEL=0 make db_bench -j64 && ./db_bench --benchmarks="readseq,readseq,readseq,readseq,readseq" --merge_operator="max" --num=100000 --db="/dev/shm/merge-random-10-operands-10K-10KB" --cache_size=1000000000 --use_existing_db --disable_auto_compactions [FullMergeV2] readseq : 24.325 micros/op 41109 ops/sec; 402.1 MB/s readseq : 1.470 micros/op 680272 ops/sec; 6653.7 MB/s readseq : 1.231 micros/op 812347 ops/sec; 7945.5 MB/s readseq : 1.091 micros/op 916590 ops/sec; 8965.1 MB/s readseq : 1.109 micros/op 901713 ops/sec; 8819.6 MB/s [master] readseq : 27.257 micros/op 36687 ops/sec; 358.8 MB/s readseq : 4.443 micros/op 225073 ops/sec; 2201.4 MB/s readseq : 5.830 micros/op 171526 ops/sec; 1677.7 MB/s readseq : 4.173 micros/op 239635 ops/sec; 2343.8 MB/s readseq : 4.150 micros/op 240963 ops/sec; 2356.8 MB/s ``` Test Plan: COMPILE_WITH_ASAN=1 make check -j64 Reviewers: yhchiang, andrewkr, sdong Reviewed By: sdong Subscribers: lovro, andrewkr, dhruba Differential Revision: https://reviews.facebook.net/D57075
9 years ago · 68a8e6b8fa
parent e70ba4e40e
commit 68a8e6b8fa
38 changed files with 814 additions and 237 deletions
--- a/db/c.cc
+++ b/db/c.cc
@ -269,33 +269,31 @@ struct rocksdb_mergeoperator_t : public MergeOperator {

  virtual const char* Name() const override { return (*name_)(state_); }

-  virtual bool FullMerge(const Slice& key, const Slice* existing_value,
-                         const std::deque<std::string>& operand_list,
-                         std::string* new_value,
-                         Logger* logger) const override {
-    size_t n = operand_list.size();
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override {
+    size_t n = merge_in.operand_list.size();
    std::vector<const char*> operand_pointers(n);
    std::vector<size_t> operand_sizes(n);
    for (size_t i = 0; i < n; i++) {
-      Slice operand(operand_list[i]);
+      Slice operand(merge_in.operand_list[i]);
      operand_pointers[i] = operand.data();
      operand_sizes[i] = operand.size();
    }

    const char* existing_value_data = nullptr;
    size_t existing_value_len = 0;
-    if (existing_value != nullptr) {
-      existing_value_data = existing_value->data();
-      existing_value_len = existing_value->size();
+    if (merge_in.existing_value != nullptr) {
+      existing_value_data = merge_in.existing_value->data();
+      existing_value_len = merge_in.existing_value->size();
    }

    unsigned char success;
    size_t new_value_len;
    char* tmp_new_value = (*full_merge_)(
-        state_, key.data(), key.size(), existing_value_data, existing_value_len,
-        &operand_pointers[0], &operand_sizes[0], static_cast<int>(n), &success,
-        &new_value_len);
-    new_value->assign(tmp_new_value, new_value_len);
+        state_, merge_in.key.data(), merge_in.key.size(), existing_value_data,
+        existing_value_len, &operand_pointers[0], &operand_sizes[0],
+        static_cast<int>(n), &success, &new_value_len);
+    merge_out->new_value.assign(tmp_new_value, new_value_len);

    if (delete_value_ != nullptr) {
      (*delete_value_)(state_, tmp_new_value, new_value_len);
--- a/db/compaction_iterator.cc
+++ b/db/compaction_iterator.cc
@ -49,6 +49,12 @@ CompactionIterator::CompactionIterator(
  } else {
    ignore_snapshots_ = false;
  }
+  input_->SetPinnedItersMgr(&pinned_iters_mgr_);
+}
+
+CompactionIterator::~CompactionIterator() {
+  // input_ Iteartor lifetime is longer than pinned_iters_mgr_ lifetime
+  input_->SetPinnedItersMgr(nullptr);
 }

 void CompactionIterator::ResetRecordCounts() {
@ -83,6 +89,8 @@ void CompactionIterator::Next() {
      ikey_.user_key = current_key_.GetUserKey();
      valid_ = true;
    } else {
+      // We consumed all pinned merge operands, release pinned iterators
+      pinned_iters_mgr_.ReleasePinnedIterators();
      // MergeHelper moves the iterator to the first record after the merged
      // records, so even though we reached the end of the merge output, we do
      // not want to advance the iterator.
@ -368,6 +376,7 @@ void CompactionIterator::NextFromInput() {
        return;
      }

+      pinned_iters_mgr_.StartPinning();
      // 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
@ -395,6 +404,7 @@ void CompactionIterator::NextFromInput() {
        // batch consumed by the merge operator should not shadow any keys
        // coming after the merges
        has_current_user_key_ = false;
+        pinned_iters_mgr_.ReleasePinnedIterators();
      }
    } else {
      valid_ = true;
--- a/db/compaction_iterator.h
+++ b/db/compaction_iterator.h
@ -13,6 +13,7 @@

 #include "db/compaction.h"
 #include "db/merge_helper.h"
+#include "db/pinned_iterators_manager.h"
 #include "rocksdb/compaction_filter.h"
 #include "util/log_buffer.h"

@ -46,6 +47,8 @@ class CompactionIterator {
                     const CompactionFilter* compaction_filter = nullptr,
                     LogBuffer* log_buffer = nullptr);

+  ~CompactionIterator();
+
  void ResetRecordCounts();

  // Seek to the beginning of the compaction iterator output.
@ -136,6 +139,9 @@ class CompactionIterator {
  bool clear_and_output_next_key_ = false;

  MergeOutputIterator merge_out_iter_;
+  // PinnedIteratorsManager used to pin input_ Iterator blocks while reading
+  // merge operands and then releasing them after consuming them.
+  PinnedIteratorsManager pinned_iters_mgr_;
  std::string compaction_filter_value_;
  // "level_ptrs" holds indices that remember which file of an associated
  // level we were last checking during the last call to compaction->
--- a/db/db_iter.cc
+++ b/db/db_iter.cc
@ -155,7 +155,9 @@ class DBIter: public Iterator {
  virtual Slice value() const override {
    assert(valid_);
    if (current_entry_is_merged_) {
-      return saved_value_;
+      // If pinned_value_ is set then the result of merge operator is one of
+      // the merge operands and we should return it.
+      return pinned_value_.data() ? pinned_value_ : saved_value_;
    } else if (direction_ == kReverse) {
      return pinned_value_;
    } else {
@ -286,9 +288,9 @@ inline bool DBIter::ParseKey(ParsedInternalKey* ikey) {
 void DBIter::Next() {
  assert(valid_);

+  // Release temporarily pinned blocks from last operation
+  ReleaseTempPinnedData();
  if (direction_ == kReverse) {
-    // We only pin blocks when doing kReverse
-    ReleaseTempPinnedData();
    FindNextUserKey();
    direction_ = kForward;
    if (!iter_->Valid()) {
@ -433,9 +435,12 @@ void DBIter::MergeValuesNewToOld() {
    return;
  }

+  // Temporarily pin the blocks that hold merge operands
+  TempPinData();
  merge_context_.Clear();
  // Start the merge process by pushing the first operand
-  merge_context_.PushOperand(iter_->value());
+  merge_context_.PushOperand(iter_->value(),
+                             iter_->IsValuePinned() /* operand_pinned */);

  ParsedInternalKey ikey;
  for (iter_->Next(); iter_->Valid(); iter_->Next()) {
@ -459,15 +464,15 @@ void DBIter::MergeValuesNewToOld() {
      const Slice val = iter_->value();
      MergeHelper::TimedFullMerge(merge_operator_, ikey.user_key, &val,
                                  merge_context_.GetOperands(), &saved_value_,
-                                  logger_, statistics_, env_);
+                                  logger_, statistics_, env_, &pinned_value_);
      // iter_ is positioned after put
      iter_->Next();
      return;
    } else if (kTypeMerge == ikey.type) {
      // hit a merge, add the value as an operand and run associative merge.
      // when complete, add result to operands and continue.
-      const Slice& val = iter_->value();
-      merge_context_.PushOperand(val);
+      merge_context_.PushOperand(iter_->value(),
+                                 iter_->IsValuePinned() /* operand_pinned */);
    } else {
      assert(false);
    }
@ -479,15 +484,15 @@ void DBIter::MergeValuesNewToOld() {
  // client can differentiate this scenario and do things accordingly.
  MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetKey(), nullptr,
                              merge_context_.GetOperands(), &saved_value_,
-                              logger_, statistics_, env_);
+                              logger_, statistics_, env_, &pinned_value_);
 }

 void DBIter::Prev() {
  assert(valid_);
+  ReleaseTempPinnedData();
  if (direction_ == kForward) {
    ReverseToBackward();
  }
-  ReleaseTempPinnedData();
  PrevInternal();
  if (statistics_ != nullptr) {
    local_stats_.prev_count_++;
@ -580,6 +585,9 @@ bool DBIter::FindValueForCurrentKey() {
  ParsedInternalKey ikey;
  FindParseableKey(&ikey, kReverse);

+  // Temporarily pin blocks that hold (merge operands / the value)
+  ReleaseTempPinnedData();
+  TempPinData();
  size_t num_skipped = 0;
  while (iter_->Valid() && ikey.sequence <= sequence_ &&
         user_comparator_->Equal(ikey.user_key, saved_key_.GetKey())) {
@ -592,8 +600,7 @@ bool DBIter::FindValueForCurrentKey() {
    switch (last_key_entry_type) {
      case kTypeValue:
        merge_context_.Clear();
-        ReleaseTempPinnedData();
-        TempPinData();
+        assert(iter_->IsValuePinned());
        pinned_value_ = iter_->value();
        last_not_merge_type = kTypeValue;
        break;
@ -605,7 +612,8 @@ bool DBIter::FindValueForCurrentKey() {
        break;
      case kTypeMerge:
        assert(merge_operator_ != nullptr);
-        merge_context_.PushOperandBack(iter_->value());
+        merge_context_.PushOperandBack(
+            iter_->value(), iter_->IsValuePinned() /* operand_pinned */);
        break;
      default:
        assert(false);
@ -628,13 +636,14 @@ bool DBIter::FindValueForCurrentKey() {
      if (last_not_merge_type == kTypeDeletion) {
        MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetKey(),
                                    nullptr, merge_context_.GetOperands(),
-                                    &saved_value_, logger_, statistics_, env_);
+                                    &saved_value_, logger_, statistics_, env_,
+                                    &pinned_value_);
      } else {
        assert(last_not_merge_type == kTypeValue);
        MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetKey(),
                                    &pinned_value_,
                                    merge_context_.GetOperands(), &saved_value_,
-                                    logger_, statistics_, env_);
+                                    logger_, statistics_, env_, &pinned_value_);
      }
      break;
    case kTypeValue:
@ -651,6 +660,9 @@ bool DBIter::FindValueForCurrentKey() {
 // This function is used in FindValueForCurrentKey.
 // We use Seek() function instead of Prev() to find necessary value
 bool DBIter::FindValueForCurrentKeyUsingSeek() {
+  // FindValueForCurrentKey will enable pinning before calling
+  // FindValueForCurrentKeyUsingSeek()
+  assert(pinned_iters_mgr_.PinningEnabled());
  std::string last_key;
  AppendInternalKey(&last_key, ParsedInternalKey(saved_key_.GetKey(), sequence_,
                                                 kValueTypeForSeek));
@ -664,8 +676,7 @@ bool DBIter::FindValueForCurrentKeyUsingSeek() {
  if (ikey.type == kTypeValue || ikey.type == kTypeDeletion ||
      ikey.type == kTypeSingleDeletion) {
    if (ikey.type == kTypeValue) {
-      ReleaseTempPinnedData();
-      TempPinData();
+      assert(iter_->IsValuePinned());
      pinned_value_ = iter_->value();
      valid_ = true;
      return true;
@ -681,7 +692,8 @@ bool DBIter::FindValueForCurrentKeyUsingSeek() {
  while (iter_->Valid() &&
         user_comparator_->Equal(ikey.user_key, saved_key_.GetKey()) &&
         ikey.type == kTypeMerge) {
-    merge_context_.PushOperand(iter_->value());
+    merge_context_.PushOperand(iter_->value(),
+                               iter_->IsValuePinned() /* operand_pinned */);
    iter_->Next();
    FindParseableKey(&ikey, kForward);
  }
@ -691,7 +703,7 @@ bool DBIter::FindValueForCurrentKeyUsingSeek() {
      ikey.type == kTypeDeletion || ikey.type == kTypeSingleDeletion) {
    MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetKey(), nullptr,
                                merge_context_.GetOperands(), &saved_value_,
-                                logger_, statistics_, env_);
+                                logger_, statistics_, env_, &pinned_value_);
    // Make iter_ valid and point to saved_key_
    if (!iter_->Valid() ||
        !user_comparator_->Equal(ikey.user_key, saved_key_.GetKey())) {
@ -705,7 +717,7 @@ bool DBIter::FindValueForCurrentKeyUsingSeek() {
  const Slice& val = iter_->value();
  MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetKey(), &val,
                              merge_context_.GetOperands(), &saved_value_,
-                              logger_, statistics_, env_);
+                              logger_, statistics_, env_, &pinned_value_);
  valid_ = true;
  return true;
 }
--- a/db/db_iter_test.cc
+++ b/db/db_iter_test.cc
@ -150,6 +150,9 @@ class TestIterator : public InternalIterator {
    return Status::OK();
  }

+  virtual bool IsKeyPinned() const override { return true; }
+  virtual bool IsValuePinned() const override { return true; }
+
 private:
  bool initialized_;
  bool valid_;
--- a/db/db_test.cc
+++ b/db/db_test.cc
@ -4848,12 +4848,11 @@ class DelayedMergeOperator : public MergeOperator {

 public:
  explicit DelayedMergeOperator(DBTest* d) : db_test_(d) {}
-  virtual bool FullMerge(const Slice& key, const Slice* existing_value,
-                         const std::deque<std::string>& operand_list,
-                         std::string* new_value,
-                         Logger* logger) const override {
+
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override {
    db_test_->env_->addon_time_.fetch_add(1000);
-    *new_value = "";
+    merge_out->new_value = "";
    return true;
  }

--- a/db/db_test2.cc
+++ b/db/db_test2.cc
@ -1487,6 +1487,267 @@ TEST_F(DBTest2, SyncPointMarker) {
  rocksdb::SyncPoint::GetInstance()->DisableProcessing();
 }
 #endif
+
+class MergeOperatorPinningTest : public DBTest2,
+                                 public testing::WithParamInterface<bool> {
+ public:
+  MergeOperatorPinningTest() { disable_block_cache_ = GetParam(); }
+
+  bool disable_block_cache_;
+};
+
+INSTANTIATE_TEST_CASE_P(MergeOperatorPinningTest, MergeOperatorPinningTest,
+                        ::testing::Bool());
+
+#ifndef ROCKSDB_LITE
+TEST_P(MergeOperatorPinningTest, OperandsMultiBlocks) {
+  Options options = CurrentOptions();
+  BlockBasedTableOptions table_options;
+  table_options.block_size = 1;  // every block will contain one entry
+  table_options.no_block_cache = disable_block_cache_;
+  options.table_factory.reset(NewBlockBasedTableFactory(table_options));
+  options.merge_operator = MergeOperators::CreateStringAppendTESTOperator();
+  options.level0_slowdown_writes_trigger = (1 << 30);
+  options.level0_stop_writes_trigger = (1 << 30);
+  options.disable_auto_compactions = true;
+  DestroyAndReopen(options);
+
+  const int kKeysPerFile = 10;
+  const int kOperandsPerKeyPerFile = 7;
+  const int kOperandSize = 100;
+  // Filse to write in L0 before compacting to lower level
+  const int kFilesPerLevel = 3;
+
+  Random rnd(301);
+  std::map<std::string, std::string> true_data;
+  int batch_num = 1;
+  int lvl_to_fill = 4;
+  int key_id = 0;
+  while (true) {
+    for (int j = 0; j < kKeysPerFile; j++) {
+      std::string key = Key(key_id % 35);
+      key_id++;
+      for (int k = 0; k < kOperandsPerKeyPerFile; k++) {
+        std::string val = RandomString(&rnd, kOperandSize);
+        ASSERT_OK(db_->Merge(WriteOptions(), key, val));
+        if (true_data[key].size() == 0) {
+          true_data[key] = val;
+        } else {
+          true_data[key] += "," + val;
+        }
+      }
+    }
+
+    if (lvl_to_fill == -1) {
+      // Keep last batch in memtable and stop
+      break;
+    }
+
+    ASSERT_OK(Flush());
+    if (batch_num % kFilesPerLevel == 0) {
+      if (lvl_to_fill != 0) {
+        MoveFilesToLevel(lvl_to_fill);
+      }
+      lvl_to_fill--;
+    }
+    batch_num++;
+  }
+
+  // 3 L0 files
+  // 1 L1 file
+  // 3 L2 files
+  // 1 L3 file
+  // 3 L4 Files
+  ASSERT_EQ(FilesPerLevel(), "3,1,3,1,3");
+
+  // Verify Get()
+  for (auto kv : true_data) {
+    ASSERT_EQ(Get(kv.first), kv.second);
+  }
+
+  Iterator* iter = db_->NewIterator(ReadOptions());
+
+  // Verify Iterator::Next()
+  auto data_iter = true_data.begin();
+  for (iter->SeekToFirst(); iter->Valid(); iter->Next(), data_iter++) {
+    ASSERT_EQ(iter->key().ToString(), data_iter->first);
+    ASSERT_EQ(iter->value().ToString(), data_iter->second);
+  }
+  ASSERT_EQ(data_iter, true_data.end());
+
+  // Verify Iterator::Prev()
+  auto data_rev = true_data.rbegin();
+  for (iter->SeekToLast(); iter->Valid(); iter->Prev(), data_rev++) {
+    ASSERT_EQ(iter->key().ToString(), data_rev->first);
+    ASSERT_EQ(iter->value().ToString(), data_rev->second);
+  }
+  ASSERT_EQ(data_rev, true_data.rend());
+
+  // Verify Iterator::Seek()
+  for (auto kv : true_data) {
+    iter->Seek(kv.first);
+    ASSERT_EQ(kv.first, iter->key().ToString());
+    ASSERT_EQ(kv.second, iter->value().ToString());
+  }
+
+  delete iter;
+}
+
+TEST_P(MergeOperatorPinningTest, Randomized) {
+  do {
+    Options options = CurrentOptions();
+    options.merge_operator = MergeOperators::CreateMaxOperator();
+    BlockBasedTableOptions table_options;
+    table_options.no_block_cache = disable_block_cache_;
+    options.table_factory.reset(NewBlockBasedTableFactory(table_options));
+    DestroyAndReopen(options);
+
+    Random rnd(301);
+    std::map<std::string, std::string> true_data;
+
+    const int kTotalMerges = 10000;
+    // Every key gets ~10 operands
+    const int kKeyRange = kTotalMerges / 10;
+    const int kOperandSize = 20;
+    const int kNumPutBefore = kKeyRange / 10;  // 10% value
+    const int kNumPutAfter = kKeyRange / 10;   // 10% overwrite
+    const int kNumDelete = kKeyRange / 10;     // 10% delete
+
+    // kNumPutBefore keys will have base values
+    for (int i = 0; i < kNumPutBefore; i++) {
+      std::string key = Key(rnd.Next() % kKeyRange);
+      std::string value = RandomString(&rnd, kOperandSize);
+      ASSERT_OK(db_->Put(WriteOptions(), key, value));
+
+      true_data[key] = value;
+    }
+
+    // Do kTotalMerges merges
+    for (int i = 0; i < kTotalMerges; i++) {
+      std::string key = Key(rnd.Next() % kKeyRange);
+      std::string value = RandomString(&rnd, kOperandSize);
+      ASSERT_OK(db_->Merge(WriteOptions(), key, value));
+
+      if (true_data[key] < value) {
+        true_data[key] = value;
+      }
+    }
+
+    // Overwrite random kNumPutAfter keys
+    for (int i = 0; i < kNumPutAfter; i++) {
+      std::string key = Key(rnd.Next() % kKeyRange);
+      std::string value = RandomString(&rnd, kOperandSize);
+      ASSERT_OK(db_->Put(WriteOptions(), key, value));
+
+      true_data[key] = value;
+    }
+
+    // Delete random kNumDelete keys
+    for (int i = 0; i < kNumDelete; i++) {
+      std::string key = Key(rnd.Next() % kKeyRange);
+      ASSERT_OK(db_->Delete(WriteOptions(), key));
+
+      true_data.erase(key);
+    }
+
+    VerifyDBFromMap(true_data);
+
+    // Skip HashCuckoo since it does not support merge operators
+  } while (ChangeOptions(kSkipMergePut | kSkipHashCuckoo));
+}
+
+class MergeOperatorHook : public MergeOperator {
+ public:
+  explicit MergeOperatorHook(std::shared_ptr<MergeOperator> _merge_op)
+      : merge_op_(_merge_op) {}
+
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override {
+    before_merge_();
+    bool res = merge_op_->FullMergeV2(merge_in, merge_out);
+    after_merge_();
+    return res;
+  }
+
+  virtual const char* Name() const override { return merge_op_->Name(); }
+
+  std::shared_ptr<MergeOperator> merge_op_;
+  std::function<void()> before_merge_ = []() {};
+  std::function<void()> after_merge_ = []() {};
+};
+
+TEST_P(MergeOperatorPinningTest, EvictCacheBeforeMerge) {
+  Options options = CurrentOptions();
+
+  auto merge_hook =
+      std::make_shared<MergeOperatorHook>(MergeOperators::CreateMaxOperator());
+  options.merge_operator = merge_hook;
+  options.disable_auto_compactions = true;
+  options.level0_slowdown_writes_trigger = (1 << 30);
+  options.level0_stop_writes_trigger = (1 << 30);
+  options.max_open_files = 20;
+  BlockBasedTableOptions bbto;
+  bbto.no_block_cache = disable_block_cache_;
+  if (bbto.no_block_cache == false) {
+    bbto.block_cache = NewLRUCache(64 * 1024 * 1024);
+  } else {
+    bbto.block_cache = nullptr;
+  }
+  options.table_factory.reset(NewBlockBasedTableFactory(bbto));
+  DestroyAndReopen(options);
+
+  const int kNumOperands = 30;
+  const int kNumKeys = 1000;
+  const int kOperandSize = 100;
+  Random rnd(301);
+
+  // 1000 keys every key have 30 operands, every operand is in a different file
+  std::map<std::string, std::string> true_data;
+  for (int i = 0; i < kNumOperands; i++) {
+    for (int j = 0; j < kNumKeys; j++) {
+      std::string k = Key(j);
+      std::string v = RandomString(&rnd, kOperandSize);
+      ASSERT_OK(db_->Merge(WriteOptions(), k, v));
+
+      true_data[k] = std::max(true_data[k], v);
+    }
+    ASSERT_OK(Flush());
+  }
+
+  std::vector<uint64_t> file_numbers = ListTableFiles(env_, dbname_);
+  ASSERT_EQ(file_numbers.size(), kNumOperands);
+  int merge_cnt = 0;
+
+  // Code executed before merge operation
+  merge_hook->before_merge_ = [&]() {
+    // Evict all tables from cache before every merge operation
+    for (uint64_t num : file_numbers) {
+      TableCache::Evict(dbfull()->TEST_table_cache(), num);
+    }
+    // Decrease cache capacity to force all unrefed blocks to be evicted
+    if (bbto.block_cache) {
+      bbto.block_cache->SetCapacity(1);
+    }
+    merge_cnt++;
+  };
+
+  // Code executed after merge operation
+  merge_hook->after_merge_ = [&]() {
+    // Increase capacity again after doing the merge
+    if (bbto.block_cache) {
+      bbto.block_cache->SetCapacity(64 * 1024 * 1024);
+    }
+  };
+
+  VerifyDBFromMap(true_data);
+  ASSERT_EQ(merge_cnt, kNumKeys * 4 /* get + next + prev + seek */);
+
+  db_->CompactRange(CompactRangeOptions(), nullptr, nullptr);
+
+  VerifyDBFromMap(true_data);
+}
+#endif  // ROCKSDB_LITE
+
 }  // namespace rocksdb

 int main(int argc, char** argv) {
--- a/db/db_test_util.cc
+++ b/db/db_test_util.cc
@ -1074,6 +1074,40 @@ std::vector<std::uint64_t> DBTestBase::ListTableFiles(Env* env,
  return file_numbers;
 }

+void DBTestBase::VerifyDBFromMap(std::map<std::string, std::string> true_data) {
+  for (auto& kv : true_data) {
+    ASSERT_EQ(Get(kv.first), kv.second);
+  }
+
+  ReadOptions ro;
+  ro.total_order_seek = true;
+  Iterator* iter = db_->NewIterator(ro);
+  // Verify Iterator::Next()
+  auto data_iter = true_data.begin();
+  for (iter->SeekToFirst(); iter->Valid(); iter->Next(), data_iter++) {
+    ASSERT_EQ(iter->key().ToString(), data_iter->first);
+    ASSERT_EQ(iter->value().ToString(), data_iter->second);
+  }
+  ASSERT_EQ(data_iter, true_data.end());
+
+  // Verify Iterator::Prev()
+  auto data_rev = true_data.rbegin();
+  for (iter->SeekToLast(); iter->Valid(); iter->Prev(), data_rev++) {
+    ASSERT_EQ(iter->key().ToString(), data_rev->first);
+    ASSERT_EQ(iter->value().ToString(), data_rev->second);
+  }
+  ASSERT_EQ(data_rev, true_data.rend());
+
+  // Verify Iterator::Seek()
+  for (auto kv : true_data) {
+    iter->Seek(kv.first);
+    ASSERT_EQ(kv.first, iter->key().ToString());
+    ASSERT_EQ(kv.second, iter->value().ToString());
+  }
+
+  delete iter;
+}
+
 #ifndef ROCKSDB_LITE

 Status DBTestBase::GenerateAndAddExternalFile(const Options options,
--- a/db/db_test_util.h
+++ b/db/db_test_util.h
@ -797,6 +797,8 @@ class DBTestBase : public testing::Test {

  std::vector<std::uint64_t> ListTableFiles(Env* env, const std::string& path);

+  void VerifyDBFromMap(std::map<std::string, std::string> true_data);
+
 #ifndef ROCKSDB_LITE
  Status GenerateAndAddExternalFile(const Options options,
                                    std::vector<int> keys, size_t file_id);
--- a/db/memtable.cc
+++ b/db/memtable.cc
@ -218,12 +218,13 @@ const char* EncodeKey(std::string* scratch, const Slice& target) {

 class MemTableIterator : public InternalIterator {
 public:
-  MemTableIterator(
-      const MemTable& mem, const ReadOptions& read_options, Arena* arena)
+  MemTableIterator(const MemTable& mem, const ReadOptions& read_options,
+                   Arena* arena)
      : bloom_(nullptr),
        prefix_extractor_(mem.prefix_extractor_),
        valid_(false),
-        arena_mode_(arena != nullptr) {
+        arena_mode_(arena != nullptr),
+        value_pinned_(!mem.GetMemTableOptions()->inplace_update_support) {
    if (prefix_extractor_ != nullptr && !read_options.total_order_seek) {
      bloom_ = mem.prefix_bloom_.get();
      iter_ = mem.table_->GetDynamicPrefixIterator(arena);
@ -306,12 +307,18 @@ class MemTableIterator : public InternalIterator {
    return true;
  }

+  virtual bool IsValuePinned() const override {
+    // memtable value is always pinned, except if we allow inplace update.
+    return value_pinned_;
+  }
+
 private:
  DynamicBloom* bloom_;
  const SliceTransform* const prefix_extractor_;
  MemTableRep::Iterator* iter_;
  bool valid_;
  bool arena_mode_;
+  bool value_pinned_;

  // No copying allowed
  MemTableIterator(const MemTableIterator&);
@ -508,7 +515,6 @@ static bool SaveValue(void* arg, const char* entry) {
      case kTypeDeletion:
      case kTypeSingleDeletion: {
        if (*(s->merge_in_progress)) {
-          *(s->status) = Status::OK();
          *(s->status) = MergeHelper::TimedFullMerge(
              merge_operator, s->key->user_key(), nullptr,
              merge_context->GetOperands(), s->value, s->logger, s->statistics,
@ -532,7 +538,8 @@ static bool SaveValue(void* arg, const char* entry) {
        }
        Slice v = GetLengthPrefixedSlice(key_ptr + key_length);
        *(s->merge_in_progress) = true;
-        merge_context->PushOperand(v);
+        merge_context->PushOperand(
+            v, s->inplace_update_support == false /* operand_pinned */);
        return true;
      }
      default:
--- a/db/merge_context.h
+++ b/db/merge_context.h
@ -4,70 +4,113 @@
 //  of patent rights can be found in the PATENTS file in the same directory.
 //
 #pragma once
+#include <string>
+#include <vector>
 #include "db/dbformat.h"
 #include "rocksdb/slice.h"
-#include <string>
-#include <deque>

 namespace rocksdb {

-const std::deque<std::string> empty_operand_list;
+const std::vector<Slice> empty_operand_list;

 // The merge context for merging a user key.
 // When doing a Get(), DB will create such a class and pass it when
 // issuing Get() operation to memtables and version_set. The operands
 // will be fetched from the context when issuing partial of full merge.
 class MergeContext {
-public:
+ public:
  // Clear all the operands
  void Clear() {
-    if (operand_list) {
-      operand_list->clear();
+    if (operand_list_) {
+      operand_list_->clear();
+      copied_operands_->clear();
    }
  }
-  // Replace all operands with merge_result, which are expected to be the
-  // merge result of them.
-  void PushPartialMergeResult(std::string& merge_result) {
-    assert (operand_list);
-    operand_list->clear();
-    operand_list->push_front(std::move(merge_result));
-  }
+
  // Push a merge operand
-  void PushOperand(const Slice& operand_slice) {
+  void PushOperand(const Slice& operand_slice, bool operand_pinned = false) {
    Initialize();
-    operand_list->push_front(operand_slice.ToString());
+    SetDirectionBackward();
+
+    if (operand_pinned) {
+      operand_list_->push_back(operand_slice);
+    } else {
+      // We need to have our own copy of the operand since it's not pinned
+      copied_operands_->emplace_back(operand_slice.data(),
+                                     operand_slice.size());
+      operand_list_->push_back(copied_operands_->back());
+    }
  }
+
  // Push back a merge operand
-  void PushOperandBack(const Slice& operand_slice) {
+  void PushOperandBack(const Slice& operand_slice,
+                       bool operand_pinned = false) {
    Initialize();
-    operand_list->push_back(operand_slice.ToString());
+    SetDirectionForward();
+
+    if (operand_pinned) {
+      operand_list_->push_back(operand_slice);
+    } else {
+      // We need to have our own copy of the operand since it's not pinned
+      copied_operands_->emplace_back(operand_slice.data(),
+                                     operand_slice.size());
+      operand_list_->push_back(copied_operands_->back());
+    }
  }
+
  // return total number of operands in the list
  size_t GetNumOperands() const {
-    if (!operand_list) {
+    if (!operand_list_) {
      return 0;
    }
-    return operand_list->size();
+    return operand_list_->size();
  }
+
  // Get the operand at the index.
-  Slice GetOperand(int index) const {
-    assert (operand_list);
-    return (*operand_list)[index];
+  Slice GetOperand(int index) {
+    assert(operand_list_);
+
+    SetDirectionForward();
+    return (*operand_list_)[index];
  }
+
  // Return all the operands.
-  const std::deque<std::string>& GetOperands() const {
-    if (!operand_list) {
+  const std::vector<Slice>& GetOperands() {
+    if (!operand_list_) {
      return empty_operand_list;
    }
-    return *operand_list;
+
+    SetDirectionForward();
+    return *operand_list_;
  }
-private:
+
+ private:
  void Initialize() {
-    if (!operand_list) {
-      operand_list.reset(new std::deque<std::string>());
+    if (!operand_list_) {
+      operand_list_.reset(new std::vector<Slice>());
+      copied_operands_.reset(new std::vector<std::string>());
    }
  }
-  std::unique_ptr<std::deque<std::string>> operand_list;
+
+  void SetDirectionForward() {
+    if (operands_reversed_ == true) {
+      std::reverse(operand_list_->begin(), operand_list_->end());
+      operands_reversed_ = false;
+    }
+  }
+
+  void SetDirectionBackward() {
+    if (operands_reversed_ == false) {
+      std::reverse(operand_list_->begin(), operand_list_->end());
+      operands_reversed_ = true;
+    }
+  }
+
+  // List of operands
+  std::unique_ptr<std::vector<Slice>> operand_list_;
+  // Copy of operands that are not pinned.
+  std::unique_ptr<std::vector<std::string>> copied_operands_;
+  bool operands_reversed_ = true;
 };

 } // namespace rocksdb
--- a/db/merge_helper.cc
+++ b/db/merge_helper.cc
@ -20,9 +20,10 @@ namespace rocksdb {

 Status MergeHelper::TimedFullMerge(const MergeOperator* merge_operator,
                                   const Slice& key, const Slice* value,
-                                   const std::deque<std::string>& operands,
+                                   const std::vector<Slice>& operands,
                                   std::string* result, Logger* logger,
-                                   Statistics* statistics, Env* env) {
+                                   Statistics* statistics, Env* env,
+                                   Slice* result_operand) {
  assert(merge_operator != nullptr);

  if (operands.size() == 0) {
@ -32,13 +33,28 @@ Status MergeHelper::TimedFullMerge(const MergeOperator* merge_operator,
  }

  bool success;
+  Slice tmp_result_operand(nullptr, 0);
+  const MergeOperator::MergeOperationInput merge_in(key, value, operands,
+                                                    logger);
+  MergeOperator::MergeOperationOutput merge_out(*result, tmp_result_operand);
  {
    // Setup to time the merge
    StopWatchNano timer(env, statistics != nullptr);
    PERF_TIMER_GUARD(merge_operator_time_nanos);

    // Do the merge
-    success = merge_operator->FullMerge(key, value, operands, result, logger);
+    success = merge_operator->FullMergeV2(merge_in, &merge_out);
+
+    if (tmp_result_operand.data()) {
+      // FullMergeV2 result is an existing operand
+      if (result_operand != nullptr) {
+        *result_operand = tmp_result_operand;
+      } else {
+        result->assign(tmp_result_operand.data(), tmp_result_operand.size());
+      }
+    } else if (result_operand) {
+      *result_operand = Slice(nullptr, 0);
+    }

    RecordTick(statistics, MERGE_OPERATION_TOTAL_TIME,
               statistics ? timer.ElapsedNanos() : 0);
@ -65,7 +81,7 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
  // Also maintain the list of merge operands seen.
  assert(HasOperator());
  keys_.clear();
-  operands_.clear();
+  merge_context_.Clear();
  assert(user_merge_operator_);
  bool first_key = true;

@ -87,7 +103,7 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
  bool hit_the_next_user_key = false;
  for (; iter->Valid(); iter->Next(), original_key_is_iter = false) {
    ParsedInternalKey ikey;
-    assert(keys_.size() == operands_.size());
+    assert(keys_.size() == merge_context_.GetNumOperands());

    if (!ParseInternalKey(iter->key(), &ikey)) {
      // stop at corrupted key
@ -142,7 +158,8 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
      const Slice* val_ptr = (kTypeValue == ikey.type) ? &val : nullptr;
      std::string merge_result;
      s = TimedFullMerge(user_merge_operator_, ikey.user_key, val_ptr,
-                         operands_, &merge_result, logger_, stats_, env_);
+                         merge_context_.GetOperands(), &merge_result, logger_,
+                         stats_, env_);

      // We store the result in keys_.back() and operands_.back()
      // if nothing went wrong (i.e.: no operand corruption on disk)
@ -152,9 +169,9 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
        orig_ikey.type = kTypeValue;
        UpdateInternalKey(&original_key, orig_ikey.sequence, orig_ikey.type);
        keys_.clear();
-        operands_.clear();
+        merge_context_.Clear();
        keys_.emplace_front(std::move(original_key));
-        operands_.emplace_front(std::move(merge_result));
+        merge_context_.PushOperand(merge_result);
      }

      // move iter to the next entry
@ -188,12 +205,13 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
          // original_key before
          ParseInternalKey(keys_.back(), &orig_ikey);
        }
-        operands_.push_front(value_slice.ToString());
+        merge_context_.PushOperand(value_slice,
+                                   iter->IsValuePinned() /* operand_pinned */);
      }
    }
  }

-  if (operands_.size() == 0) {
+  if (merge_context_.GetNumOperands() == 0) {
    // we filtered out all the merge operands
    return Status::OK();
  }
@ -218,11 +236,12 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
    // do a final merge with nullptr as the existing value and say
    // bye to the merge type (it's now converted to a Put)
    assert(kTypeMerge == orig_ikey.type);
-    assert(operands_.size() >= 1);
-    assert(operands_.size() == keys_.size());
+    assert(merge_context_.GetNumOperands() >= 1);
+    assert(merge_context_.GetNumOperands() == keys_.size());
    std::string merge_result;
    s = TimedFullMerge(user_merge_operator_, orig_ikey.user_key, nullptr,
-                       operands_, &merge_result, logger_, stats_, env_);
+                       merge_context_.GetOperands(), &merge_result, logger_,
+                       stats_, env_);
    if (s.ok()) {
      // The original key encountered
      // We are certain that keys_ is not empty here (see assertions couple of
@ -231,9 +250,9 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
      orig_ikey.type = kTypeValue;
      UpdateInternalKey(&original_key, orig_ikey.sequence, orig_ikey.type);
      keys_.clear();
-      operands_.clear();
+      merge_context_.Clear();
      keys_.emplace_front(std::move(original_key));
-      operands_.emplace_front(std::move(merge_result));
+      merge_context_.PushOperand(merge_result);
    }
  } else {
    // We haven't seen the beginning of the key nor a Put/Delete.
@ -244,8 +263,8 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
    // partial merge returns Status::OK(). Should we change the status code
    // after a successful partial merge?
    s = Status::MergeInProgress();
-    if (operands_.size() >= 2 &&
-        operands_.size() >= min_partial_merge_operands_) {
+    if (merge_context_.GetNumOperands() >= 2 &&
+        merge_context_.GetNumOperands() >= min_partial_merge_operands_) {
      bool merge_success = false;
      std::string merge_result;
      {
@ -253,7 +272,8 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
        PERF_TIMER_GUARD(merge_operator_time_nanos);
        merge_success = user_merge_operator_->PartialMergeMulti(
            orig_ikey.user_key,
-            std::deque<Slice>(operands_.begin(), operands_.end()),
+            std::deque<Slice>(merge_context_.GetOperands().begin(),
+                              merge_context_.GetOperands().end()),
            &merge_result, logger_);
        RecordTick(stats_, MERGE_OPERATION_TOTAL_TIME,
                   stats_ ? timer.ElapsedNanosSafe() : 0);
@ -261,8 +281,8 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
      if (merge_success) {
        // Merging of operands (associative merge) was successful.
        // Replace operands with the merge result
-        operands_.clear();
-        operands_.emplace_front(std::move(merge_result));
+        merge_context_.Clear();
+        merge_context_.PushOperand(merge_result);
        keys_.erase(keys_.begin(), keys_.end() - 1);
      }
    }
--- a/db/merge_helper.h
+++ b/db/merge_helper.h
@ -8,8 +8,10 @@

 #include <deque>
 #include <string>
+#include <vector>

 #include "db/dbformat.h"
+#include "db/merge_context.h"
 #include "rocksdb/compaction_filter.h"
 #include "rocksdb/env.h"
 #include "rocksdb/slice.h"
@ -42,14 +44,13 @@ class MergeHelper {
        latest_snapshot_(latest_snapshot),
        level_(level),
        keys_(),
-        operands_(),
        filter_timer_(env_),
        total_filter_time_(0U),
        stats_(stats) {
    assert(user_comparator_ != nullptr);
  }

-  // Wrapper around MergeOperator::FullMerge() that records perf statistics.
+  // Wrapper around MergeOperator::FullMergeV2() that records perf statistics.
  // Result of merge will be written to result if status returned is OK.
  // If operands is empty, the value will simply be copied to result.
  // Returns one of the following statuses:
@ -57,9 +58,10 @@ class MergeHelper {
  // - Corruption: Merge operator reported unsuccessful merge.
  static Status TimedFullMerge(const MergeOperator* merge_operator,
                               const Slice& key, const Slice* value,
-                               const std::deque<std::string>& operands,
+                               const std::vector<Slice>& operands,
                               std::string* result, Logger* logger,
-                               Statistics* statistics, Env* env);
+                               Statistics* statistics, Env* env,
+                               Slice* result_operand = nullptr);

  // Merge entries until we hit
  //     - a corrupted key
@ -116,7 +118,9 @@ class MergeHelper {
  //                So keys().back() was the first key seen by iterator.
  // TODO: Re-style this comment to be like the first one
  const std::deque<std::string>& keys() const { return keys_; }
-  const std::deque<std::string>& values() const { return operands_; }
+  const std::vector<Slice>& values() const {
+    return merge_context_.GetOperands();
+  }
  uint64_t TotalFilterTime() const { return total_filter_time_; }
  bool HasOperator() const { return user_merge_operator_ != nullptr; }

@ -133,8 +137,11 @@ class MergeHelper {

  // the scratch area that holds the result of MergeUntil
  // valid up to the next MergeUntil call
-  std::deque<std::string> keys_;    // Keeps track of the sequence of keys seen
-  std::deque<std::string> operands_;  // Parallel with keys_; stores the values
+
+  // Keeps track of the sequence of keys seen
+  std::deque<std::string> keys_;
+  // Parallel with keys_; stores the operands
+  mutable MergeContext merge_context_;

  StopWatchNano filter_timer_;
  uint64_t total_filter_time_;
@ -159,7 +166,7 @@ class MergeOutputIterator {
 private:
  const MergeHelper* merge_helper_;
  std::deque<std::string>::const_reverse_iterator it_keys_;
-  std::deque<std::string>::const_reverse_iterator it_values_;
+  std::vector<Slice>::const_reverse_iterator it_values_;
 };

 } // namespace rocksdb
--- a/db/merge_operator.cc
+++ b/db/merge_operator.cc
@ -11,6 +11,18 @@

 namespace rocksdb {

+bool MergeOperator::FullMergeV2(const MergeOperationInput& merge_in,
+                                MergeOperationOutput* merge_out) const {
+  // If FullMergeV2 is not implemented, we convert the operand_list to
+  // std::deque<std::string> and pass it to FullMerge
+  std::deque<std::string> operand_list_str;
+  for (auto& op : merge_in.operand_list) {
+    operand_list_str.emplace_back(op.data(), op.size());
+  }
+  return FullMerge(merge_in.key, merge_in.existing_value, operand_list_str,
+                   &merge_out->new_value, merge_in.logger);
+}
+
 // The default implementation of PartialMergeMulti, which invokes
 // PartialMerge multiple times internally and merges two operands at
 // a time.
@ -39,23 +51,20 @@ bool MergeOperator::PartialMergeMulti(const Slice& key,
 // Given a "real" merge from the library, call the user's
 // associative merge function one-by-one on each of the operands.
 // NOTE: It is assumed that the client's merge-operator will handle any errors.
-bool AssociativeMergeOperator::FullMerge(
-    const Slice& key,
-    const Slice* existing_value,
-    const std::deque<std::string>& operand_list,
-    std::string* new_value,
-    Logger* logger) const {
-
+bool AssociativeMergeOperator::FullMergeV2(
+    const MergeOperationInput& merge_in,
+    MergeOperationOutput* merge_out) const {
  // Simply loop through the operands
  Slice temp_existing;
-  for (const auto& operand : operand_list) {
-    Slice value(operand);
+  const Slice* existing_value = merge_in.existing_value;
+  for (const auto& operand : merge_in.operand_list) {
    std::string temp_value;
-    if (!Merge(key, existing_value, value, &temp_value, logger)) {
+    if (!Merge(merge_in.key, existing_value, operand, &temp_value,
+               merge_in.logger)) {
      return false;
    }
-    swap(temp_value, *new_value);
-    temp_existing = Slice(*new_value);
+    swap(temp_value, merge_out->new_value);
+    temp_existing = Slice(merge_out->new_value);
    existing_value = &temp_existing;
  }

--- a/db/pinned_iterators_manager.h
+++ b/db/pinned_iterators_manager.h
@ -18,7 +18,7 @@ namespace rocksdb {
 class PinnedIteratorsManager {
 public:
  PinnedIteratorsManager() : pinning_enabled(false), pinned_iters_(nullptr) {}
-  ~PinnedIteratorsManager() { assert(!pinning_enabled); }
+  ~PinnedIteratorsManager() { ReleasePinnedIterators(); }

  // Enable Iterators pinning
  void StartPinning() {
@ -43,7 +43,7 @@ class PinnedIteratorsManager {
  }

  // Release pinned Iterators
-  void ReleasePinnedIterators() {
+  inline void ReleasePinnedIterators() {
    if (pinning_enabled) {
      pinning_enabled = false;

--- a/db/version_set.cc
+++ b/db/version_set.cc
@ -31,6 +31,7 @@
 #include "db/memtable.h"
 #include "db/merge_context.h"
 #include "db/merge_helper.h"
+#include "db/pinned_iterators_manager.h"
 #include "db/table_cache.h"
 #include "db/version_builder.h"
 #include "rocksdb/env.h"
@ -917,10 +918,17 @@ void Version::Get(const ReadOptions& read_options, const LookupKey& k,
    *key_exists = true;
  }

+  PinnedIteratorsManager pinned_iters_mgr;
  GetContext get_context(
      user_comparator(), merge_operator_, info_log_, db_statistics_,
      status->ok() ? GetContext::kNotFound : GetContext::kMerge, user_key,
-      value, value_found, merge_context, this->env_, seq);
+      value, value_found, merge_context, this->env_, seq,
+      merge_operator_ ? &pinned_iters_mgr : nullptr);
+
+  // Pin blocks that we read to hold merge operands
+  if (merge_operator_) {
+    pinned_iters_mgr.StartPinning();
+  }

  FilePicker fp(
      storage_info_.files_, user_key, ikey, &storage_info_.level_files_brief_,
--- a/db/write_batch.cc
+++ b/db/write_batch.cc
@ -922,12 +922,10 @@ class MemTableInserter : public WriteBatch::Handler {
      auto merge_operator = moptions->merge_operator;
      assert(merge_operator);

-      std::deque<std::string> operands;
-      operands.push_front(value.ToString());
      std::string new_value;

      Status merge_status = MergeHelper::TimedFullMerge(
-          merge_operator, key, &get_value_slice, operands, &new_value,
+          merge_operator, key, &get_value_slice, {value}, &new_value,
          moptions->info_log, moptions->statistics, Env::Default());

      if (!merge_status.ok()) {
--- a/examples/compaction_filter_example.cc
+++ b/examples/compaction_filter_example.cc
@ -10,19 +10,18 @@

 class MyMerge : public rocksdb::MergeOperator {
 public:
-  bool FullMerge(const rocksdb::Slice& key,
-                 const rocksdb::Slice* existing_value,
-                 const std::deque<std::string>& operand_list,
-                 std::string* new_value,
-                 rocksdb::Logger* logger) const override {
-    new_value->clear();
-    if (existing_value != nullptr) {
-      new_value->assign(existing_value->data(), existing_value->size());
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override {
+    merge_out->new_value.clear();
+    if (merge_in.existing_value != nullptr) {
+      merge_out->new_value.assign(merge_in.existing_value->data(),
+                                  merge_in.existing_value->size());
    }
-    for (const std::string& m : operand_list) {
-      fprintf(stderr, "Merge(%s)\n", m.c_str());
-      assert(m != "bad");  // the compaction filter filters out bad values
-      new_value->assign(m);
+    for (const rocksdb::Slice& m : merge_in.operand_list) {
+      fprintf(stderr, "Merge(%s)\n", m.ToString().c_str());
+      // the compaction filter filters out bad values
+      assert(m.ToString() != "bad");
+      merge_out->new_value.assign(m.data(), m.size());
    }
    return true;
  }
--- a/include/rocksdb/merge_operator.h
+++ b/include/rocksdb/merge_operator.h
@ -9,6 +9,7 @@
 #include <deque>
 #include <memory>
 #include <string>
+#include <vector>

 #include "rocksdb/slice.h"

@ -32,7 +33,7 @@ class Logger;
 //    into rocksdb); numeric addition and string concatenation are examples;
 //
 //  b) MergeOperator - the generic class for all the more abstract / complex
-//    operations; one method (FullMerge) to merge a Put/Delete value with a
+//    operations; one method (FullMergeV2) to merge a Put/Delete value with a
 //    merge operand; and another method (PartialMerge) that merges multiple
 //    operands together. this is especially useful if your key values have
 //    complex structures but you would still like to support client-specific
@ -69,7 +70,49 @@ class MergeOperator {
                         const Slice* existing_value,
                         const std::deque<std::string>& operand_list,
                         std::string* new_value,
-                         Logger* logger) const = 0;
+                         Logger* logger) const {
+    // deprecated, please use FullMergeV2()
+    assert(false);
+    return false;
+  }
+
+  struct MergeOperationInput {
+    explicit MergeOperationInput(const Slice& _key,
+                                 const Slice* _existing_value,
+                                 const std::vector<Slice>& _operand_list,
+                                 Logger* _logger)
+        : key(_key),
+          existing_value(_existing_value),
+          operand_list(_operand_list),
+          logger(_logger) {}
+
+    // The key associated with the merge operation.
+    const Slice& key;
+    // The existing value of the current key, nullptr means that the
+    // value dont exist.
+    const Slice* existing_value;
+    // A list of operands to apply.
+    const std::vector<Slice>& operand_list;
+    // Logger could be used by client to log any errors that happen during
+    // the merge operation.
+    Logger* logger;
+  };
+
+  struct MergeOperationOutput {
+    explicit MergeOperationOutput(std::string& _new_value,
+                                  Slice& _existing_operand)
+        : new_value(_new_value), existing_operand(_existing_operand) {}
+
+    // Client is responsible for filling the merge result here.
+    std::string& new_value;
+    // If the merge result is one of the existing operands (or existing_value),
+    // client can set this field to the operand (or existing_value) instead of
+    // using new_value.
+    Slice& existing_operand;
+  };
+
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const;

  // This function performs merge(left_op, right_op)
  // when both the operands are themselves merge operation types
@ -99,7 +142,7 @@ class MergeOperator {
  // TODO: Presently there is no way to differentiate between error/corruption
  // and simply "return false". For now, the client should simply return
  // false in any case it cannot perform partial-merge, regardless of reason.
-  // If there is corruption in the data, handle it in the FullMerge() function,
+  // If there is corruption in the data, handle it in the FullMergeV2() function
  // and return false there.  The default implementation of PartialMerge will
  // always return false.
  virtual bool PartialMerge(const Slice& key, const Slice& left_operand,
@ -171,11 +214,8 @@ class AssociativeMergeOperator : public MergeOperator {

 private:
  // Default implementations of the MergeOperator functions
-  virtual bool FullMerge(const Slice& key,
-                         const Slice* existing_value,
-                         const std::deque<std::string>& operand_list,
-                         std::string* new_value,
-                         Logger* logger) const override;
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override;

  virtual bool PartialMerge(const Slice& key,
                            const Slice& left_operand,
--- a/table/block.h
+++ b/table/block.h
@ -162,6 +162,8 @@ class BlockIter : public InternalIterator {

  virtual bool IsKeyPinned() const override { return key_pinned_; }

+  virtual bool IsValuePinned() const override { return true; }
+
 private:
  const Comparator* comparator_;
  const char* data_;       // underlying block contents
--- a/table/block_based_table_reader.cc
+++ b/table/block_based_table_reader.cc
@ -12,6 +12,7 @@
 #include <utility>

 #include "db/dbformat.h"
+#include "db/pinned_iterators_manager.h"

 #include "rocksdb/cache.h"
 #include "rocksdb/comparator.h"
@ -1381,6 +1382,10 @@ Status BlockBasedTable::Get(const ReadOptions& read_options, const Slice& key,
    BlockIter iiter;
    NewIndexIterator(read_options, &iiter);

+    PinnedIteratorsManager* pinned_iters_mgr = get_context->pinned_iters_mgr();
+    bool pin_blocks = pinned_iters_mgr && pinned_iters_mgr->PinningEnabled();
+    BlockIter* biter = nullptr;
+
    bool done = false;
    for (iiter.Seek(key); iiter.Valid() && !done; iiter.Next()) {
      Slice handle_value = iiter.value();
@ -1398,37 +1403,60 @@ Status BlockBasedTable::Get(const ReadOptions& read_options, const Slice& key,
        RecordTick(rep_->ioptions.statistics, BLOOM_FILTER_USEFUL);
        break;
      } else {
-        BlockIter biter;
-        NewDataBlockIterator(rep_, read_options, iiter.value(), &biter);
+        BlockIter stack_biter;
+        if (pin_blocks) {
+          // We need to create the BlockIter on heap because we may need to
+          // pin it if we encounterd merge operands
+          biter = static_cast<BlockIter*>(
+              NewDataBlockIterator(rep_, read_options, iiter.value()));
+        } else {
+          biter = &stack_biter;
+          NewDataBlockIterator(rep_, read_options, iiter.value(), biter);
+        }

        if (read_options.read_tier == kBlockCacheTier &&
-            biter.status().IsIncomplete()) {
+            biter->status().IsIncomplete()) {
          // couldn't get block from block_cache
          // Update Saver.state to Found because we are only looking for whether
          // we can guarantee the key is not there when "no_io" is set
          get_context->MarkKeyMayExist();
          break;
        }
-        if (!biter.status().ok()) {
-          s = biter.status();
+        if (!biter->status().ok()) {
+          s = biter->status();
          break;
        }

        // Call the *saver function on each entry/block until it returns false
-        for (biter.Seek(key); biter.Valid(); biter.Next()) {
+        for (biter->Seek(key); biter->Valid(); biter->Next()) {
          ParsedInternalKey parsed_key;
-          if (!ParseInternalKey(biter.key(), &parsed_key)) {
+          if (!ParseInternalKey(biter->key(), &parsed_key)) {
            s = Status::Corruption(Slice());
          }

-          if (!get_context->SaveValue(parsed_key, biter.value())) {
+          if (!get_context->SaveValue(parsed_key, biter->value(), pin_blocks)) {
            done = true;
            break;
          }
        }
-        s = biter.status();
+        s = biter->status();
+
+        if (pin_blocks) {
+          if (get_context->State() == GetContext::kMerge) {
+            // Pin blocks as long as we are merging
+            pinned_iters_mgr->PinIteratorIfNeeded(biter);
+          } else {
+            delete biter;
+          }
+          biter = nullptr;
+        } else {
+          // biter is on stack, Nothing to clean
+        }
      }
    }
+    if (pin_blocks && biter != nullptr) {
+      delete biter;
+    }
    if (s.ok()) {
      s = iiter.status();
    }
--- a/table/get_context.cc
+++ b/table/get_context.cc
@ -5,6 +5,7 @@

 #include "table/get_context.h"
 #include "db/merge_helper.h"
+#include "db/pinned_iterators_manager.h"
 #include "rocksdb/env.h"
 #include "rocksdb/merge_operator.h"
 #include "rocksdb/statistics.h"
@ -36,7 +37,8 @@ GetContext::GetContext(const Comparator* ucmp,
                       Statistics* statistics, GetState init_state,
                       const Slice& user_key, std::string* ret_value,
                       bool* value_found, MergeContext* merge_context, Env* env,
-                       SequenceNumber* seq)
+                       SequenceNumber* seq,
+                       PinnedIteratorsManager* _pinned_iters_mgr)
    : ucmp_(ucmp),
      merge_operator_(merge_operator),
      logger_(logger),
@ -48,7 +50,8 @@ GetContext::GetContext(const Comparator* ucmp,
      merge_context_(merge_context),
      env_(env),
      seq_(seq),
-      replay_log_(nullptr) {
+      replay_log_(nullptr),
+      pinned_iters_mgr_(_pinned_iters_mgr) {
  if (seq_) {
    *seq_ = kMaxSequenceNumber;
  }
@ -77,7 +80,7 @@ void GetContext::SaveValue(const Slice& value, SequenceNumber seq) {
 }

 bool GetContext::SaveValue(const ParsedInternalKey& parsed_key,
-                           const Slice& value) {
+                           const Slice& value, bool value_pinned) {
  assert((state_ != kMerge && parsed_key.type != kTypeMerge) ||
         merge_context_ != nullptr);
  if (ucmp_->Equal(parsed_key.user_key, user_key_)) {
@ -139,7 +142,7 @@ bool GetContext::SaveValue(const ParsedInternalKey& parsed_key,
      case kTypeMerge:
        assert(state_ == kNotFound || state_ == kMerge);
        state_ = kMerge;
-        merge_context_->PushOperand(value);
+        merge_context_->PushOperand(value, value_pinned);
        return true;

      default:
@ -167,7 +170,7 @@ void replayGetContextLog(const Slice& replay_log, const Slice& user_key,
    // Since SequenceNumber is not stored and unknown, we will use
    // kMaxSequenceNumber.
    get_context->SaveValue(
-        ParsedInternalKey(user_key, kMaxSequenceNumber, type), value);
+        ParsedInternalKey(user_key, kMaxSequenceNumber, type), value, true);
  }
 #else   // ROCKSDB_LITE
  assert(false);
--- a/table/get_context.h
+++ b/table/get_context.h
@ -11,6 +11,7 @@

 namespace rocksdb {
 class MergeContext;
+class PinnedIteratorsManager;

 class GetContext {
 public:
@ -26,7 +27,8 @@ class GetContext {
             Logger* logger, Statistics* statistics, GetState init_state,
             const Slice& user_key, std::string* ret_value, bool* value_found,
             MergeContext* merge_context, Env* env,
-             SequenceNumber* seq = nullptr);
+             SequenceNumber* seq = nullptr,
+             PinnedIteratorsManager* _pinned_iters_mgr = nullptr);

  void MarkKeyMayExist();

@ -35,7 +37,8 @@ class GetContext {
  //
  // Returns True if more keys need to be read (due to merges) or
  //         False if the complete value has been found.
-  bool SaveValue(const ParsedInternalKey& parsed_key, const Slice& value);
+  bool SaveValue(const ParsedInternalKey& parsed_key, const Slice& value,
+                 bool value_pinned = false);

  // Simplified version of the previous function. Should only be used when we
  // know that the operation is a Put.
@ -43,6 +46,8 @@ class GetContext {

  GetState State() const { return state_; }

+  PinnedIteratorsManager* pinned_iters_mgr() { return pinned_iters_mgr_; }
+
  // If a non-null string is passed, all the SaveValue calls will be
  // logged into the string. The operations can then be replayed on
  // another GetContext with replayGetContextLog.
@ -68,6 +73,8 @@ class GetContext {
  // write to the key or kMaxSequenceNumber if unknown
  SequenceNumber* seq_;
  std::string* replay_log_;
+  // Used to temporarily pin blocks when state_ == GetContext::kMerge
+  PinnedIteratorsManager* pinned_iters_mgr_;
 };

 void replayGetContextLog(const Slice& replay_log, const Slice& user_key,
--- a/table/internal_iterator.h
+++ b/table/internal_iterator.h
@ -80,6 +80,11 @@ class InternalIterator : public Cleanable {
  //    set to false.
  virtual bool IsKeyPinned() const { return false; }

+  // If true, this means that the Slice returned by value() is valid as long as
+  // PinnedIteratorsManager::ReleasePinnedIterators is not called and the
+  // Iterator is not deleted.
+  virtual bool IsValuePinned() const { return false; }
+
  virtual Status GetProperty(std::string prop_name, std::string* prop) {
    return Status::NotSupported("");
  }
--- a/table/iterator_wrapper.h
+++ b/table/iterator_wrapper.h
@ -16,7 +16,7 @@
 namespace rocksdb {

 // A internal wrapper class with an interface similar to Iterator that caches
-// the valid(), key() and IsKeyPinned() results for an underlying iterator.
+// the valid() and key() results for an underlying iterator.
 // This can help avoid virtual function calls and also gives better
 // cache locality.
 class IteratorWrapper {
@ -55,7 +55,6 @@ class IteratorWrapper {
  // Iterator interface methods
  bool Valid() const        { return valid_; }
  Slice key() const         { assert(Valid()); return key_; }
-  bool IsKeyPinned() const  { assert(Valid()); return is_key_pinned_; }
  Slice value() const       { assert(Valid()); return iter_->value(); }
  // Methods below require iter() != nullptr
  Status status() const     { assert(iter_); return iter_->status(); }
@ -64,10 +63,18 @@ class IteratorWrapper {
  void Seek(const Slice& k) { assert(iter_); iter_->Seek(k);       Update(); }
  void SeekToFirst()        { assert(iter_); iter_->SeekToFirst(); Update(); }
  void SeekToLast()         { assert(iter_); iter_->SeekToLast();  Update(); }
+
  void SetPinnedItersMgr(PinnedIteratorsManager* pinned_iters_mgr) {
    assert(iter_);
    iter_->SetPinnedItersMgr(pinned_iters_mgr);
-    Update();
+  }
+  bool IsKeyPinned() const {
+    assert(Valid());
+    return iter_->IsKeyPinned();
+  }
+  bool IsValuePinned() const {
+    assert(Valid());
+    return iter_->IsValuePinned();
  }

 private:
@ -75,14 +82,12 @@ class IteratorWrapper {
    valid_ = iter_->Valid();
    if (valid_) {
      key_ = iter_->key();
-      is_key_pinned_ = iter_->IsKeyPinned();
    }
  }

  InternalIterator* iter_;
  bool valid_;
  Slice key_;
-  bool is_key_pinned_;
 };

 class Arena;
--- a/table/merger.cc
+++ b/table/merger.cc
@ -257,6 +257,12 @@ class MergingIterator : public InternalIterator {
           current_->IsKeyPinned();
  }

+  virtual bool IsValuePinned() const override {
+    assert(Valid());
+    return pinned_iters_mgr_ && pinned_iters_mgr_->PinningEnabled() &&
+           current_->IsValuePinned();
+  }
+
 private:
  // Clears heaps for both directions, used when changing direction or seeking
  void ClearHeaps();
--- a/table/two_level_iterator.cc
+++ b/table/two_level_iterator.cc
@ -78,6 +78,10 @@ class TwoLevelIterator : public InternalIterator {
    return pinned_iters_mgr_ && pinned_iters_mgr_->PinningEnabled() &&
           second_level_iter_.iter() && second_level_iter_.IsKeyPinned();
  }
+  virtual bool IsValuePinned() const override {
+    return pinned_iters_mgr_ && pinned_iters_mgr_->PinningEnabled() &&
+           second_level_iter_.iter() && second_level_iter_.IsValuePinned();
+  }

 private:
  void SaveError(const Status& s) {
--- a/tools/db_crashtest.py
+++ b/tools/db_crashtest.py
@ -45,6 +45,8 @@ default_params = {
    "write_buffer_size": 4 * 1024 * 1024,
    "writepercent": 35,
    "subcompactions": lambda: random.randint(1, 4),
+    "use_merge": lambda: random.randint(0, 1),
+    "use_full_merge_v1": lambda: random.randint(0, 1),
 }


--- a/tools/db_stress.cc
+++ b/tools/db_stress.cc
@ -453,6 +453,9 @@ static const bool FLAGS_prefix_size_dummy __attribute__((unused)) =
 DEFINE_bool(use_merge, false, "On true, replaces all writes with a Merge "
            "that behaves like a Put");

+DEFINE_bool(use_full_merge_v1, false,
+            "On true, use a merge operator that implement the deprecated "
+            "version of FullMerge");

 namespace rocksdb {

@ -2106,7 +2109,11 @@ class StressTest {
    }

    if (FLAGS_use_merge) {
-      options_.merge_operator = MergeOperators::CreatePutOperator();
+      if (FLAGS_use_full_merge_v1) {
+        options_.merge_operator = MergeOperators::CreateDeprecatedPutOperator();
+      } else {
+        options_.merge_operator = MergeOperators::CreatePutOperator();
+      }
    }

    // set universal style compaction configurations, if applicable
--- a/util/testutil.h
+++ b/util/testutil.h
@ -614,10 +614,8 @@ class ChanglingMergeOperator : public MergeOperator {

  void SetName(const std::string& name) { name_ = name; }

-  virtual bool FullMerge(const Slice& key, const Slice* existing_value,
-                         const std::deque<std::string>& operand_list,
-                         std::string* new_value,
-                         Logger* logger) const override {
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override {
    return false;
  }
  virtual bool PartialMergeMulti(const Slice& key,
--- a/utilities/merge_operators.h
+++ b/utilities/merge_operators.h
@ -16,6 +16,7 @@ namespace rocksdb {
 class MergeOperators {
 public:
  static std::shared_ptr<MergeOperator> CreatePutOperator();
+  static std::shared_ptr<MergeOperator> CreateDeprecatedPutOperator();
  static std::shared_ptr<MergeOperator> CreateUInt64AddOperator();
  static std::shared_ptr<MergeOperator> CreateStringAppendOperator();
  static std::shared_ptr<MergeOperator> CreateStringAppendTESTOperator();
@ -27,6 +28,8 @@ class MergeOperators {
      const std::string& name) {
    if (name == "put") {
      return CreatePutOperator();
+    } else if (name == "put_v1") {
+      return CreateDeprecatedPutOperator();
    } else if ( name == "uint64add") {
      return CreateUInt64AddOperator();
    } else if (name == "stringappend") {
--- a/utilities/merge_operators/max.cc
+++ b/utilities/merge_operators/max.cc
@ -19,22 +19,20 @@ namespace {  // anonymous namespace
 // Slice::compare
 class MaxOperator : public MergeOperator {
 public:
-  virtual bool FullMerge(const Slice& key, const Slice* existing_value,
-                         const std::deque<std::string>& operand_list,
-                         std::string* new_value,
-                         Logger* logger) const override {
-    Slice max;
-    if (existing_value) {
-      max = Slice(existing_value->data(), existing_value->size());
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override {
+    Slice& max = merge_out->existing_operand;
+    if (merge_in.existing_value) {
+      max = Slice(merge_in.existing_value->data(),
+                  merge_in.existing_value->size());
    }

-    for (const auto& op : operand_list) {
+    for (const auto& op : merge_in.operand_list) {
      if (max.compare(op) < 0) {
-        max = Slice(op.data(), op.size());
+        max = op;
      }
    }

-    new_value->assign(max.data(), max.size());
    return true;
  }

--- a/utilities/merge_operators/put.cc
+++ b/utilities/merge_operators/put.cc
@ -57,12 +57,33 @@ class PutOperator : public MergeOperator {
  }
 };

+class PutOperatorV2 : public PutOperator {
+  virtual bool FullMerge(const Slice& key, const Slice* existing_value,
+                         const std::deque<std::string>& operand_sequence,
+                         std::string* new_value,
+                         Logger* logger) const override {
+    assert(false);
+    return false;
+  }
+
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override {
+    // Put basically only looks at the current/latest value
+    assert(!merge_in.operand_list.empty());
+    merge_out->existing_operand = merge_in.operand_list.back();
+    return true;
+  }
+};
+
 } // end of anonymous namespace

 namespace rocksdb {

-std::shared_ptr<MergeOperator> MergeOperators::CreatePutOperator() {
+std::shared_ptr<MergeOperator> MergeOperators::CreateDeprecatedPutOperator() {
  return std::make_shared<PutOperator>();
 }

+std::shared_ptr<MergeOperator> MergeOperators::CreatePutOperator() {
+  return std::make_shared<PutOperatorV2>();
+}
 }
--- a/utilities/merge_operators/string_append/stringappend2.cc
+++ b/utilities/merge_operators/string_append/stringappend2.cc
@ -21,20 +21,22 @@ StringAppendTESTOperator::StringAppendTESTOperator(char delim_char)
 }

 // Implementation for the merge operation (concatenates two strings)
-bool StringAppendTESTOperator::FullMerge(
-    const Slice& key,
-    const Slice* existing_value,
-    const std::deque<std::string>& operands,
-    std::string* new_value,
-    Logger* logger) const {
-
+bool StringAppendTESTOperator::FullMergeV2(
+    const MergeOperationInput& merge_in,
+    MergeOperationOutput* merge_out) const {
  // Clear the *new_value for writing.
-  assert(new_value);
-  new_value->clear();
+  merge_out->new_value.clear();
+
+  if (merge_in.existing_value == nullptr && merge_in.operand_list.size() == 1) {
+    // Only one operand
+    merge_out->existing_operand = merge_in.operand_list.back();
+    return true;
+  }

  // Compute the space needed for the final result.
  size_t numBytes = 0;
-  for(auto it = operands.begin(); it != operands.end(); ++it) {
+  for (auto it = merge_in.operand_list.begin();
+       it != merge_in.operand_list.end(); ++it) {
    numBytes += it->size() + 1;   // Plus 1 for the delimiter
  }

@ -42,20 +44,23 @@ bool StringAppendTESTOperator::FullMerge(
  bool printDelim = false;

  // Prepend the *existing_value if one exists.
-  if (existing_value) {
-    new_value->reserve(numBytes + existing_value->size());
-    new_value->append(existing_value->data(), existing_value->size());
+  if (merge_in.existing_value) {
+    merge_out->new_value.reserve(numBytes + merge_in.existing_value->size());
+    merge_out->new_value.append(merge_in.existing_value->data(),
+                                merge_in.existing_value->size());
    printDelim = true;
  } else if (numBytes) {
-    new_value->reserve(numBytes-1); // Minus 1 since we have one less delimiter
+    merge_out->new_value.reserve(
+        numBytes - 1);  // Minus 1 since we have one less delimiter
  }

  // Concatenate the sequence of strings (and add a delimiter between each)
-  for(auto it = operands.begin(); it != operands.end(); ++it) {
+  for (auto it = merge_in.operand_list.begin();
+       it != merge_in.operand_list.end(); ++it) {
    if (printDelim) {
-      new_value->append(1,delim_);
+      merge_out->new_value.append(1, delim_);
    }
-    new_value->append(*it);
+    merge_out->new_value.append(it->data(), it->size());
    printDelim = true;
  }

--- a/utilities/merge_operators/string_append/stringappend2.h
+++ b/utilities/merge_operators/string_append/stringappend2.h
@ -24,11 +24,8 @@ class StringAppendTESTOperator : public MergeOperator {
  // Constructor with delimiter
  explicit StringAppendTESTOperator(char delim_char);

-  virtual bool FullMerge(const Slice& key,
-                         const Slice* existing_value,
-                         const std::deque<std::string>& operand_sequence,
-                         std::string* new_value,
-                         Logger* logger) const override;
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override;

  virtual bool PartialMergeMulti(const Slice& key,
                                 const std::deque<Slice>& operand_list,
--- a/utilities/options/options_util_test.cc
+++ b/utilities/options/options_util_test.cc
@ -128,19 +128,19 @@ class DummyMergeOperator : public MergeOperator {
  DummyMergeOperator() {}
  virtual ~DummyMergeOperator() {}

-  virtual bool FullMerge(const Slice& key, const Slice* existing_value,
-                         const std::deque<std::string>& operand_list,
-                         std::string* new_value, Logger* logger) const {
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override {
    return false;
  }

  virtual bool PartialMergeMulti(const Slice& key,
                                 const std::deque<Slice>& operand_list,
-                                 std::string* new_value, Logger* logger) const {
+                                 std::string* new_value,
+                                 Logger* logger) const override {
    return false;
  }

-  virtual const char* Name() const { return "DummyMergeOperator"; }
+  virtual const char* Name() const override { return "DummyMergeOperator"; }
 };

 class DummySliceTransform : public SliceTransform {
--- a/utilities/ttl/db_ttl_impl.h
+++ b/utilities/ttl/db_ttl_impl.h
@ -225,38 +225,43 @@ class TtlMergeOperator : public MergeOperator {
    assert(env);
  }

-  virtual bool FullMerge(const Slice& key, const Slice* existing_value,
-                         const std::deque<std::string>& operands,
-                         std::string* new_value, Logger* logger) const
-      override {
+  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
+                           MergeOperationOutput* merge_out) const override {
    const uint32_t ts_len = DBWithTTLImpl::kTSLength;
-    if (existing_value && existing_value->size() < ts_len) {
-      Log(InfoLogLevel::ERROR_LEVEL, logger,
+    if (merge_in.existing_value && merge_in.existing_value->size() < ts_len) {
+      Log(InfoLogLevel::ERROR_LEVEL, merge_in.logger,
          "Error: Could not remove timestamp from existing value.");
      return false;
    }

    // Extract time-stamp from each operand to be passed to user_merge_op_
-    std::deque<std::string> operands_without_ts;
-    for (const auto& operand : operands) {
+    std::vector<Slice> operands_without_ts;
+    for (const auto& operand : merge_in.operand_list) {
      if (operand.size() < ts_len) {
-        Log(InfoLogLevel::ERROR_LEVEL, logger,
+        Log(InfoLogLevel::ERROR_LEVEL, merge_in.logger,
            "Error: Could not remove timestamp from operand value.");
        return false;
      }
-      operands_without_ts.push_back(operand.substr(0, operand.size() - ts_len));
+      operands_without_ts.push_back(operand);
+      operands_without_ts.back().remove_suffix(ts_len);
    }

    // Apply the user merge operator (store result in *new_value)
    bool good = true;
-    if (existing_value) {
-      Slice existing_value_without_ts(existing_value->data(),
-                                      existing_value->size() - ts_len);
-      good = user_merge_op_->FullMerge(key, &existing_value_without_ts,
-                                       operands_without_ts, new_value, logger);
+    MergeOperationOutput user_merge_out(merge_out->new_value,
+                                        merge_out->existing_operand);
+    if (merge_in.existing_value) {
+      Slice existing_value_without_ts(merge_in.existing_value->data(),
+                                      merge_in.existing_value->size() - ts_len);
+      good = user_merge_op_->FullMergeV2(
+          MergeOperationInput(merge_in.key, &existing_value_without_ts,
+                              operands_without_ts, merge_in.logger),
+          &user_merge_out);
    } else {
-      good = user_merge_op_->FullMerge(key, nullptr, operands_without_ts,
-                                       new_value, logger);
+      good = user_merge_op_->FullMergeV2(
+          MergeOperationInput(merge_in.key, nullptr, operands_without_ts,
+                              merge_in.logger),
+          &user_merge_out);
    }

    // Return false if the user merge operator returned false
@ -264,17 +269,23 @@ class TtlMergeOperator : public MergeOperator {
      return false;
    }

+    if (merge_out->existing_operand.data()) {
+      merge_out->new_value.assign(merge_out->existing_operand.data(),
+                                  merge_out->existing_operand.size());
+      merge_out->existing_operand = Slice(nullptr, 0);
+    }
+
    // Augment the *new_value with the ttl time-stamp
    int64_t curtime;
    if (!env_->GetCurrentTime(&curtime).ok()) {
-      Log(InfoLogLevel::ERROR_LEVEL, logger,
+      Log(InfoLogLevel::ERROR_LEVEL, merge_in.logger,
          "Error: Could not get current time to be attached internally "
          "to the new value.");
      return false;
    } else {
      char ts_string[ts_len];
      EncodeFixed32(ts_string, (int32_t)curtime);
-      new_value->append(ts_string, ts_len);
+      merge_out->new_value.append(ts_string, ts_len);
      return true;
    }
  }
--- a/utilities/util_merge_operators_test.cc
+++ b/utilities/util_merge_operators_test.cc
@ -13,22 +13,41 @@ class UtilMergeOperatorTest : public testing::Test {
 public:
  UtilMergeOperatorTest() {}

-  std::string FullMerge(std::string existing_value,
-                        std::deque<std::string> operands,
-                        std::string key = "") {
-    Slice existing_value_slice(existing_value);
+  std::string FullMergeV2(std::string existing_value,
+                          std::vector<std::string> operands,
+                          std::string key = "") {
    std::string result;
+    Slice result_operand(nullptr, 0);
+
+    Slice existing_value_slice(existing_value);
+    std::vector<Slice> operands_slice(operands.begin(), operands.end());

-    merge_operator_->FullMerge(key, &existing_value_slice, operands, &result,
-                               nullptr);
+    const MergeOperator::MergeOperationInput merge_in(
+        key, &existing_value_slice, operands_slice, nullptr);
+    MergeOperator::MergeOperationOutput merge_out(result, result_operand);
+    merge_operator_->FullMergeV2(merge_in, &merge_out);
+
+    if (result_operand.data()) {
+      result.assign(result_operand.data(), result_operand.size());
+    }
    return result;
  }

-  std::string FullMerge(std::deque<std::string> operands,
-                        std::string key = "") {
+  std::string FullMergeV2(std::vector<std::string> operands,
+                          std::string key = "") {
    std::string result;
+    Slice result_operand(nullptr, 0);
+
+    std::vector<Slice> operands_slice(operands.begin(), operands.end());
+
+    const MergeOperator::MergeOperationInput merge_in(key, nullptr,
+                                                      operands_slice, nullptr);
+    MergeOperator::MergeOperationOutput merge_out(result, result_operand);
+    merge_operator_->FullMergeV2(merge_in, &merge_out);

-    merge_operator_->FullMerge(key, nullptr, operands, &result, nullptr);
+    if (result_operand.data()) {
+      result.assign(result_operand.data(), result_operand.size());
+    }
    return result;
  }

@ -56,14 +75,14 @@ class UtilMergeOperatorTest : public testing::Test {
 TEST_F(UtilMergeOperatorTest, MaxMergeOperator) {
  merge_operator_ = MergeOperators::CreateMaxOperator();

-  EXPECT_EQ("B", FullMerge("B", {"A"}));
-  EXPECT_EQ("B", FullMerge("A", {"B"}));
-  EXPECT_EQ("", FullMerge({"", "", ""}));
-  EXPECT_EQ("A", FullMerge({"A"}));
-  EXPECT_EQ("ABC", FullMerge({"ABC"}));
-  EXPECT_EQ("Z", FullMerge({"ABC", "Z", "C", "AXX"}));
-  EXPECT_EQ("ZZZ", FullMerge({"ABC", "CC", "Z", "ZZZ"}));
-  EXPECT_EQ("a", FullMerge("a", {"ABC", "CC", "Z", "ZZZ"}));
+  EXPECT_EQ("B", FullMergeV2("B", {"A"}));
+  EXPECT_EQ("B", FullMergeV2("A", {"B"}));
+  EXPECT_EQ("", FullMergeV2({"", "", ""}));
+  EXPECT_EQ("A", FullMergeV2({"A"}));
+  EXPECT_EQ("ABC", FullMergeV2({"ABC"}));
+  EXPECT_EQ("Z", FullMergeV2({"ABC", "Z", "C", "AXX"}));
+  EXPECT_EQ("ZZZ", FullMergeV2({"ABC", "CC", "Z", "ZZZ"}));
+  EXPECT_EQ("a", FullMergeV2("a", {"ABC", "CC", "Z", "ZZZ"}));

  EXPECT_EQ("z", PartialMergeMulti({"a", "z", "efqfqwgwew", "aaz", "hhhhh"}));