Add an interface to provide support for underlying FS to pass their own buffer during reads (#11324)

Summary:
1. Public API change: Replace `use_async_io`  API in file_system with `SupportedOps` API which is used by underlying FileSystem to indicate to upper layers whether the FileSystem supports different operations introduced in `enum FSSupportedOps `. Right now operations are `async_io` and whether FS will provide its own buffer during reads or not. The api is changed to extend it to various FileSystem operations in one API rather than creating a separate API for each operation.

2. Provide support for underlying FS to pass their own buffer during Reads (async and sync read) instead of using RocksDB provided `scratch` (buffer) in `FSReadRequest`. Currently only MultiRead supports it and later will be extended to other reads as well (point lookup, scan etc). More details in how to enable in file_system.h

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11324

Test Plan: Tested locally

Reviewed By: anand1976

Differential Revision: D44465322

Pulled By: akankshamahajan15

fbshipit-source-id: 9ec9e08f839b5cc815e75d5dade6cd549998d0ec

db/arena_wrapped_db_iter.cc

@@ -47,7 +47,9 @@ void ArenaWrappedDBIter::Init(
   read_options_ = read_options;
   allow_refresh_ = allow_refresh;
   memtable_range_tombstone_iter_ = nullptr;
-  if (!env->GetFileSystem()->use_async_io()) {
+
+  if (!CheckFSFeatureSupport(env->GetFileSystem().get(),
+                             FSSupportedOps::kAsyncIO)) {
     read_options_.async_io = false;
   }
 }

db/blob/blob_file_reader.cc

@@ -439,11 +439,11 @@ void BlobFileReader::MultiGetBlob(
     assert(req->offset >= adjustment);
     adjustments.push_back(adjustment);
 
-    FSReadRequest read_req = {};
+    FSReadRequest read_req;
     read_req.offset = req->offset - adjustment;
     read_req.len = req->len + adjustment;
-    read_reqs.emplace_back(read_req);
     total_len += read_req.len;
+    read_reqs.emplace_back(std::move(read_req));
   }
 
   RecordTick(statistics_, BLOB_DB_BLOB_FILE_BYTES_READ, total_len);

db/db_tailing_iter_test.cc

@@ -15,6 +15,11 @@
 #include "db/forward_iterator.h"
 #include "port/stack_trace.h"
 
+namespace {
+static bool enable_io_uring = true;
+extern "C" bool RocksDbIOUringEnable() { return enable_io_uring; }
+}  // namespace
+
 namespace ROCKSDB_NAMESPACE {
 
 class DBTestTailingIterator : public DBTestBase,
@@ -50,94 +55,125 @@ TEST_P(DBTestTailingIterator, TailingIteratorSingle) {
 }
 
 TEST_P(DBTestTailingIterator, TailingIteratorKeepAdding) {
-  CreateAndReopenWithCF({"pikachu"}, CurrentOptions());
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
+  std::unique_ptr<Env> env(
+      new CompositeEnvWrapper(env_, FileSystem::Default()));
+  Options options = CurrentOptions();
+  options.env = env.get();
+  CreateAndReopenWithCF({"pikachu"}, options);
   ReadOptions read_options;
   read_options.tailing = true;
   if (GetParam()) {
     read_options.async_io = true;
   }
-  std::unique_ptr<Iterator> iter(db_->NewIterator(read_options, handles_[1]));
-  ASSERT_OK(iter->status());
-  std::string value(1024, 'a');
 
-  const int num_records = 10000;
+  {
-  for (int i = 0; i < num_records; ++i) {
+    std::unique_ptr<Iterator> iter(db_->NewIterator(read_options, handles_[1]));
-    char buf[32];
+    ASSERT_OK(iter->status());
-    snprintf(buf, sizeof(buf), "%016d", i);
+    std::string value(1024, 'a');
 
-    Slice key(buf, 16);
+    const int num_records = 10000;
-    ASSERT_OK(Put(1, key, value));
+    for (int i = 0; i < num_records; ++i) {
+      char buf[32];
+      snprintf(buf, sizeof(buf), "%016d", i);
 
-    iter->Seek(key);
+      Slice key(buf, 16);
-    ASSERT_TRUE(iter->Valid());
+      ASSERT_OK(Put(1, key, value));
-    ASSERT_EQ(iter->key().compare(key), 0);
+
+      iter->Seek(key);
+      ASSERT_TRUE(iter->Valid());
+      ASSERT_EQ(iter->key().compare(key), 0);
+    }
   }
+  Close();
 }
 
 TEST_P(DBTestTailingIterator, TailingIteratorSeekToNext) {
-  CreateAndReopenWithCF({"pikachu"}, CurrentOptions());
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
+  std::unique_ptr<Env> env(
+      new CompositeEnvWrapper(env_, FileSystem::Default()));
+  Options options = CurrentOptions();
+  options.env = env.get();
+  CreateAndReopenWithCF({"pikachu"}, options);
   ReadOptions read_options;
   read_options.tailing = true;
   if (GetParam()) {
     read_options.async_io = true;
   }
-  std::unique_ptr<Iterator> iter(db_->NewIterator(read_options, handles_[1]));
+  {
-  ASSERT_OK(iter->status());
+    std::unique_ptr<Iterator> iter(db_->NewIterator(read_options, handles_[1]));
-  std::unique_ptr<Iterator> itern(db_->NewIterator(read_options, handles_[1]));
+    ASSERT_OK(iter->status());
-  ASSERT_OK(itern->status());
+    std::unique_ptr<Iterator> itern(
-  std::string value(1024, 'a');
+        db_->NewIterator(read_options, handles_[1]));
-
+    ASSERT_OK(itern->status());
-  const int num_records = 1000;
+    std::string value(1024, 'a');
-  for (int i = 1; i < num_records; ++i) {
+
-    char buf1[32];
+    const int num_records = 1000;
-    char buf2[32];
+    for (int i = 1; i < num_records; ++i) {
-    snprintf(buf1, sizeof(buf1), "00a0%016d", i * 5);
+      char buf1[32];
-
+      char buf2[32];
-    Slice key(buf1, 20);
+      snprintf(buf1, sizeof(buf1), "00a0%016d", i * 5);
-    ASSERT_OK(Put(1, key, value));
+
+      Slice key(buf1, 20);
+      ASSERT_OK(Put(1, key, value));
+
+      if (i % 100 == 99) {
+        ASSERT_OK(Flush(1));
+      }
 
-    if (i % 100 == 99) {
+      snprintf(buf2, sizeof(buf2), "00a0%016d", i * 5 - 2);
-      ASSERT_OK(Flush(1));
+      Slice target(buf2, 20);
+      iter->Seek(target);
+      ASSERT_TRUE(iter->Valid());
+      ASSERT_EQ(iter->key().compare(key), 0);
+      if (i == 1) {
+        itern->SeekToFirst();
+      } else {
+        itern->Next();
+      }
+      ASSERT_TRUE(itern->Valid());
+      ASSERT_EQ(itern->key().compare(key), 0);
     }
+    ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->ClearAllCallBacks();
+    ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->DisableProcessing();
+    for (int i = 2 * num_records; i > 0; --i) {
+      char buf1[32];
+      char buf2[32];
+      snprintf(buf1, sizeof(buf1), "00a0%016d", i * 5);
+
+      Slice key(buf1, 20);
+      ASSERT_OK(Put(1, key, value));
+
+      if (i % 100 == 99) {
+        ASSERT_OK(Flush(1));
+      }
 
-    snprintf(buf2, sizeof(buf2), "00a0%016d", i * 5 - 2);
+      snprintf(buf2, sizeof(buf2), "00a0%016d", i * 5 - 2);
-    Slice target(buf2, 20);
+      Slice target(buf2, 20);
-    iter->Seek(target);
+      iter->Seek(target);
-    ASSERT_TRUE(iter->Valid());
+      ASSERT_TRUE(iter->Valid());
-    ASSERT_EQ(iter->key().compare(key), 0);
+      ASSERT_EQ(iter->key().compare(key), 0);
-    if (i == 1) {
-      itern->SeekToFirst();
-    } else {
-      itern->Next();
     }
-    ASSERT_TRUE(itern->Valid());
-    ASSERT_EQ(itern->key().compare(key), 0);
-  }
-  ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->ClearAllCallBacks();
-  ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->DisableProcessing();
-  for (int i = 2 * num_records; i > 0; --i) {
-    char buf1[32];
-    char buf2[32];
-    snprintf(buf1, sizeof(buf1), "00a0%016d", i * 5);
-
-    Slice key(buf1, 20);
-    ASSERT_OK(Put(1, key, value));
-
-    if (i % 100 == 99) {
-      ASSERT_OK(Flush(1));
-    }
-
-    snprintf(buf2, sizeof(buf2), "00a0%016d", i * 5 - 2);
-    Slice target(buf2, 20);
-    iter->Seek(target);
-    ASSERT_TRUE(iter->Valid());
-    ASSERT_EQ(iter->key().compare(key), 0);
   }
+  Close();
 }
 
 TEST_P(DBTestTailingIterator, TailingIteratorTrimSeekToNext) {
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
   const uint64_t k150KB = 150 * 1024;
+  std::unique_ptr<Env> env(
+      new CompositeEnvWrapper(env_, FileSystem::Default()));
   Options options;
+  options.env = env.get();
   options.write_buffer_size = k150KB;
   options.max_write_buffer_number = 3;
   options.min_write_buffer_number_to_merge = 2;
@@ -278,56 +314,73 @@ TEST_P(DBTestTailingIterator, TailingIteratorTrimSeekToNext) {
 }
 
 TEST_P(DBTestTailingIterator, TailingIteratorDeletes) {
-  CreateAndReopenWithCF({"pikachu"}, CurrentOptions());
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
+  std::unique_ptr<Env> env(
+      new CompositeEnvWrapper(env_, FileSystem::Default()));
+  Options options = CurrentOptions();
+  options.env = env.get();
+  CreateAndReopenWithCF({"pikachu"}, options);
   ReadOptions read_options;
   read_options.tailing = true;
   if (GetParam()) {
     read_options.async_io = true;
   }
+  {
+    std::unique_ptr<Iterator> iter(db_->NewIterator(read_options, handles_[1]));
+    ASSERT_OK(iter->status());
 
-  std::unique_ptr<Iterator> iter(db_->NewIterator(read_options, handles_[1]));
+    // write a single record, read it using the iterator, then delete it
-  ASSERT_OK(iter->status());
+    ASSERT_OK(Put(1, "0test", "test"));
-
+    iter->SeekToFirst();
-  // write a single record, read it using the iterator, then delete it
+    ASSERT_TRUE(iter->Valid());
-  ASSERT_OK(Put(1, "0test", "test"));
+    ASSERT_EQ(iter->key().ToString(), "0test");
-  iter->SeekToFirst();
+    ASSERT_OK(Delete(1, "0test"));
-  ASSERT_TRUE(iter->Valid());
-  ASSERT_EQ(iter->key().ToString(), "0test");
-  ASSERT_OK(Delete(1, "0test"));
 
-  // write many more records
+    // write many more records
-  const int num_records = 10000;
+    const int num_records = 10000;
-  std::string value(1024, 'A');
+    std::string value(1024, 'A');
 
-  for (int i = 0; i < num_records; ++i) {
+    for (int i = 0; i < num_records; ++i) {
-    char buf[32];
+      char buf[32];
-    snprintf(buf, sizeof(buf), "1%015d", i);
+      snprintf(buf, sizeof(buf), "1%015d", i);
 
-    Slice key(buf, 16);
+      Slice key(buf, 16);
-    ASSERT_OK(Put(1, key, value));
+      ASSERT_OK(Put(1, key, value));
-  }
+    }
 
-  // force a flush to make sure that no records are read from memtable
+    // force a flush to make sure that no records are read from memtable
-  ASSERT_OK(Flush(1));
+    ASSERT_OK(Flush(1));
 
-  // skip "0test"
+    // skip "0test"
-  iter->Next();
+    iter->Next();
 
-  // make sure we can read all new records using the existing iterator
+    // make sure we can read all new records using the existing iterator
-  int count = 0;
+    int count = 0;
-  for (; iter->Valid(); iter->Next(), ++count)
+    for (; iter->Valid(); iter->Next(), ++count)
-    ;
+      ;
 
-  ASSERT_EQ(count, num_records);
+    ASSERT_EQ(count, num_records);
+  }
+  Close();
 }
 
 TEST_P(DBTestTailingIterator, TailingIteratorPrefixSeek) {
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
   ReadOptions read_options;
   read_options.tailing = true;
   if (GetParam()) {
     read_options.async_io = true;
   }
+  std::unique_ptr<Env> env(
+      new CompositeEnvWrapper(env_, FileSystem::Default()));
   Options options = CurrentOptions();
+  options.env = env.get();
   options.create_if_missing = true;
   options.disable_auto_compactions = true;
   options.prefix_extractor.reset(NewFixedPrefixTransform(2));
@@ -336,28 +389,39 @@ TEST_P(DBTestTailingIterator, TailingIteratorPrefixSeek) {
   DestroyAndReopen(options);
   CreateAndReopenWithCF({"pikachu"}, options);
 
-  std::unique_ptr<Iterator> iter(db_->NewIterator(read_options, handles_[1]));
+  {
-  ASSERT_OK(iter->status());
+    std::unique_ptr<Iterator> iter(db_->NewIterator(read_options, handles_[1]));
-  ASSERT_OK(Put(1, "0101", "test"));
+    ASSERT_OK(iter->status());
+    ASSERT_OK(Put(1, "0101", "test"));
 
-  ASSERT_OK(Flush(1));
+    ASSERT_OK(Flush(1));
 
-  ASSERT_OK(Put(1, "0202", "test"));
+    ASSERT_OK(Put(1, "0202", "test"));
 
-  // Seek(0102) shouldn't find any records since 0202 has a different prefix
+    // Seek(0102) shouldn't find any records since 0202 has a different prefix
-  iter->Seek("0102");
+    iter->Seek("0102");
-  ASSERT_TRUE(!iter->Valid());
+    ASSERT_TRUE(!iter->Valid());
 
-  iter->Seek("0202");
+    iter->Seek("0202");
-  ASSERT_TRUE(iter->Valid());
+    ASSERT_TRUE(iter->Valid());
-  ASSERT_EQ(iter->key().ToString(), "0202");
+    ASSERT_EQ(iter->key().ToString(), "0202");
 
-  iter->Next();
+    iter->Next();
-  ASSERT_TRUE(!iter->Valid());
+    ASSERT_TRUE(!iter->Valid());
+  }
+  Close();
 }
 
 TEST_P(DBTestTailingIterator, TailingIteratorIncomplete) {
-  CreateAndReopenWithCF({"pikachu"}, CurrentOptions());
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
+  std::unique_ptr<Env> env(
+      new CompositeEnvWrapper(env_, FileSystem::Default()));
+  Options options = CurrentOptions();
+  options.env = env.get();
+  CreateAndReopenWithCF({"pikachu"}, options);
   ReadOptions read_options;
   read_options.tailing = true;
   if (GetParam()) {
@@ -370,20 +434,30 @@ TEST_P(DBTestTailingIterator, TailingIteratorIncomplete) {
 
   ASSERT_OK(db_->Put(WriteOptions(), key, value));
 
-  std::unique_ptr<Iterator> iter(db_->NewIterator(read_options));
+  {
-  ASSERT_OK(iter->status());
+    std::unique_ptr<Iterator> iter(db_->NewIterator(read_options));
-  iter->SeekToFirst();
+    ASSERT_OK(iter->status());
-  // we either see the entry or it's not in cache
+    iter->SeekToFirst();
-  ASSERT_TRUE(iter->Valid() || iter->status().IsIncomplete());
+    // we either see the entry or it's not in cache
-
+    ASSERT_TRUE(iter->Valid() || iter->status().IsIncomplete());
-  ASSERT_OK(db_->CompactRange(CompactRangeOptions(), nullptr, nullptr));
+
-  iter->SeekToFirst();
+    ASSERT_OK(db_->CompactRange(CompactRangeOptions(), nullptr, nullptr));
-  // should still be true after compaction
+    iter->SeekToFirst();
-  ASSERT_TRUE(iter->Valid() || iter->status().IsIncomplete());
+    // should still be true after compaction
+    ASSERT_TRUE(iter->Valid() || iter->status().IsIncomplete());
+  }
+  Close();
 }
 
 TEST_P(DBTestTailingIterator, TailingIteratorSeekToSame) {
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
+  std::unique_ptr<Env> env(
+      new CompositeEnvWrapper(env_, FileSystem::Default()));
   Options options = CurrentOptions();
+  options.env = env.get();
   options.compaction_style = kCompactionStyleUniversal;
   options.write_buffer_size = 1000;
   CreateAndReopenWithCF({"pikachu"}, options);
@@ -403,28 +477,39 @@ TEST_P(DBTestTailingIterator, TailingIteratorSeekToSame) {
     ASSERT_OK(db_->Put(WriteOptions(), key, value));
   }
 
-  std::unique_ptr<Iterator> iter(db_->NewIterator(read_options));
+  {
-  ASSERT_OK(iter->status());
+    std::unique_ptr<Iterator> iter(db_->NewIterator(read_options));
-  // Seek to 00001.  We expect to find 00002.
+    ASSERT_OK(iter->status());
-  std::string start_key = "00001";
+    // Seek to 00001.  We expect to find 00002.
-  iter->Seek(start_key);
+    std::string start_key = "00001";
-  ASSERT_TRUE(iter->Valid());
+    iter->Seek(start_key);
+    ASSERT_TRUE(iter->Valid());
 
-  std::string found = iter->key().ToString();
+    std::string found = iter->key().ToString();
-  ASSERT_EQ("00002", found);
+    ASSERT_EQ("00002", found);
 
-  // Now seek to the same key.  The iterator should remain in the same
+    // Now seek to the same key.  The iterator should remain in the same
-  // position.
+    // position.
-  iter->Seek(found);
+    iter->Seek(found);
-  ASSERT_TRUE(iter->Valid());
+    ASSERT_TRUE(iter->Valid());
-  ASSERT_EQ(found, iter->key().ToString());
+    ASSERT_EQ(found, iter->key().ToString());
+  }
+  Close();
 }
 
 // Sets iterate_upper_bound and verifies that ForwardIterator doesn't call
 // Seek() on immutable iterators when target key is >= prev_key and all
 // iterators, including the memtable iterator, are over the upper bound.
 TEST_P(DBTestTailingIterator, TailingIteratorUpperBound) {
-  CreateAndReopenWithCF({"pikachu"}, CurrentOptions());
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
+  std::unique_ptr<Env> env(
+      new CompositeEnvWrapper(env_, FileSystem::Default()));
+  Options options = CurrentOptions();
+  options.env = env.get();
+  CreateAndReopenWithCF({"pikachu"}, options);
 
   const Slice upper_bound("20", 3);
   ReadOptions read_options;
@@ -441,34 +526,51 @@ TEST_P(DBTestTailingIterator, TailingIteratorUpperBound) {
   // Add another key to the memtable.
   ASSERT_OK(Put(1, "21", "21"));
 
-  std::unique_ptr<Iterator> it(db_->NewIterator(read_options, handles_[1]));
+  {
-  ASSERT_OK(it->status());
+    bool read_async_called = false;
-  it->Seek("12");
+
-  ASSERT_TRUE(it->Valid());
+    SyncPoint::GetInstance()->SetCallBack(
-  ASSERT_EQ("12", it->key().ToString());
+        "UpdateResults::io_uring_result",
-
+        [&](void* /*arg*/) { read_async_called = true; });
-  it->Next();
+    ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->EnableProcessing();
-  // Not valid since "21" is over the upper bound.
+
-  ASSERT_FALSE(it->Valid());
+    auto it =
-  ASSERT_OK(it->status());
+        std::unique_ptr<Iterator>(db_->NewIterator(read_options, handles_[1]));
-  // This keeps track of the number of times NeedToSeekImmutable() was true.
+    ASSERT_OK(it->status());
-  int immutable_seeks = 0;
+    it->Seek("12");
-  ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->SetCallBack(
+    ASSERT_TRUE(it->Valid());
-      "ForwardIterator::SeekInternal:Immutable",
+    ASSERT_EQ("12", it->key().ToString());
-      [&](void* /*arg*/) { ++immutable_seeks; });
+
-
+    it->Next();
-  // Seek to 13. This should not require any immutable seeks.
+    // Not valid since "21" is over the upper bound.
-  ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->EnableProcessing();
+    ASSERT_FALSE(it->Valid());
-  it->Seek("13");
+    ASSERT_OK(it->status());
-  ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->DisableProcessing();
+    // This keeps track of the number of times NeedToSeekImmutable() was true.
-
+    int immutable_seeks = 0;
-  ASSERT_FALSE(it->Valid());
+    ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->SetCallBack(
-  ASSERT_OK(it->status());
+        "ForwardIterator::SeekInternal:Immutable",
-  if (GetParam()) {
+        [&](void* /*arg*/) { ++immutable_seeks; });
-    ASSERT_EQ(1, immutable_seeks);
+
-  } else {
+    // Seek to 13. This should not require any immutable seeks.
-    ASSERT_EQ(0, immutable_seeks);
+    ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->EnableProcessing();
+    it->Seek("13");
+    ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->DisableProcessing();
+    ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->ClearAllCallBacks();
+
+    SyncPoint::GetInstance()->SetCallBack(
+        "UpdateResults::io_uring_result",
+        [&](void* /*arg*/) { read_async_called = true; });
+    ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->EnableProcessing();
+
+    ASSERT_FALSE(it->Valid());
+    ASSERT_OK(it->status());
+    if (GetParam() && read_async_called) {
+      ASSERT_EQ(1, immutable_seeks);
+    } else {
+      ASSERT_EQ(0, immutable_seeks);
+    }
   }
+  Close();
 }
 
 TEST_P(DBTestTailingIterator, TailingIteratorGap) {
@@ -480,7 +582,15 @@ TEST_P(DBTestTailingIterator, TailingIteratorGap) {
   // the largest key of index n file and the smallest key of index n+1 file
   // if both file fit in that gap. In this example, 25 < key < 35
   // https://github.com/facebook/rocksdb/issues/1372
-  CreateAndReopenWithCF({"pikachu"}, CurrentOptions());
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
+  std::unique_ptr<Env> env(
+      new CompositeEnvWrapper(env_, FileSystem::Default()));
+  Options options = CurrentOptions();
+  options.env = env.get();
+  CreateAndReopenWithCF({"pikachu"}, options);
 
   ReadOptions read_options;
   read_options.tailing = true;
@@ -512,20 +622,23 @@ TEST_P(DBTestTailingIterator, TailingIteratorGap) {
   ColumnFamilyMetaData meta;
   db_->GetColumnFamilyMetaData(handles_[1], &meta);
 
-  std::unique_ptr<Iterator> it(db_->NewIterator(read_options, handles_[1]));
+  {
-  it->Seek("30");
+    std::unique_ptr<Iterator> it(db_->NewIterator(read_options, handles_[1]));
-  ASSERT_TRUE(it->Valid());
+    it->Seek("30");
-  ASSERT_EQ("30", it->key().ToString());
+    ASSERT_TRUE(it->Valid());
+    ASSERT_EQ("30", it->key().ToString());
 
-  it->Next();
+    it->Next();
-  ASSERT_TRUE(it->Valid());
+    ASSERT_TRUE(it->Valid());
-  ASSERT_EQ("35", it->key().ToString());
+    ASSERT_EQ("35", it->key().ToString());
 
-  it->Next();
+    it->Next();
-  ASSERT_TRUE(it->Valid());
+    ASSERT_TRUE(it->Valid());
-  ASSERT_EQ("40", it->key().ToString());
+    ASSERT_EQ("40", it->key().ToString());
 
-  ASSERT_OK(it->status());
+    ASSERT_OK(it->status());
+  }
+  Close();
 }
 
 TEST_P(DBTestTailingIterator, SeekWithUpperBoundBug) {

db/forward_iterator.cc

@@ -241,10 +241,10 @@ ForwardIterator::ForwardIterator(DBImpl* db, const ReadOptions& read_options,
   if (sv_) {
     RebuildIterators(false);
   }
-  if (!cfd_->ioptions()->env->GetFileSystem()->use_async_io()) {
+  if (!CheckFSFeatureSupport(cfd_->ioptions()->env->GetFileSystem().get(),
+                             FSSupportedOps::kAsyncIO)) {
     read_options_.async_io = false;
   }
-
   // immutable_status_ is a local aggregation of the
   // status of the immutable Iterators.
   // We have to PermitUncheckedError in case it is never
@@ -1067,4 +1067,3 @@ void ForwardIterator::DeleteIterator(InternalIterator* iter, bool is_arena) {
 }
 
 }  // namespace ROCKSDB_NAMESPACE
-

db/version_set.cc

@@ -39,6 +39,7 @@
 #include "db/version_builder.h"
 #include "db/version_edit.h"
 #include "db/version_edit_handler.h"
+#include "file/file_util.h"
 #include "table/compaction_merging_iterator.h"
 
 #if USE_COROUTINES
@@ -2190,7 +2191,12 @@ Version::Version(ColumnFamilyData* column_family_data, VersionSet* vset,
           MaxFileSizeForL0MetaPin(mutable_cf_options_)),
       version_number_(version_number),
       io_tracer_(io_tracer),
-      use_async_io_(env_->GetFileSystem()->use_async_io()) {}
+      use_async_io_(false) {
+  if (CheckFSFeatureSupport(env_->GetFileSystem().get(),
+                            FSSupportedOps::kAsyncIO)) {
+    use_async_io_ = true;
+  }
+}
 
 Status Version::GetBlob(const ReadOptions& read_options, const Slice& user_key,
                         const Slice& blob_index_slice,

env/env.cc

@@ -159,8 +159,7 @@ class LegacyRandomAccessFileWrapper : public FSRandomAccessFile {
       req.len = fs_reqs[i].len;
       req.scratch = fs_reqs[i].scratch;
       req.status = Status::OK();
-
+      reqs.emplace_back(std::move(req));
-      reqs.emplace_back(req);
     }
     status = target_->MultiRead(reqs.data(), num_reqs);
     for (size_t i = 0; i < num_reqs; ++i) {

env/env_test.cc

@@ -3538,7 +3538,7 @@ IOStatus ReadAsyncRandomAccessFile::ReadAsync(
         }
       };
 
-  fs_.workers.emplace_back(submit_request, req);
+  fs_.workers.emplace_back(submit_request, std::move(req));
   return IOStatus::OK();
 }
 

env/fs_posix.cc

@@ -1183,11 +1183,13 @@ class PosixFileSystem : public FileSystem {
 #endif
   }
 
-  bool use_async_io() override {
+  void SupportedOps(int64_t& supported_ops) override {
+    supported_ops = 0;
 #if defined(ROCKSDB_IOURING_PRESENT)
-    return IsIOUringEnabled();
+    if (IsIOUringEnabled()) {
-#else
+      // Underlying FS supports async_io
-    return false;
+      supported_ops |= (1 << FSSupportedOps::kAsyncIO);
+    }
 #endif
   }
 

file/file_util.h

@@ -88,4 +88,14 @@ inline IOStatus PrepareIOFromReadOptions(const ReadOptions& ro,
 // Test method to delete the input directory and all of its contents.
 // This method is destructive and is meant for use only in tests!!!
 Status DestroyDir(Env* env, const std::string& dir);
+
+inline bool CheckFSFeatureSupport(FileSystem* fs, FSSupportedOps feat) {
+  int64_t supported_ops = 0;
+  fs->SupportedOps(supported_ops);
+  if (supported_ops & (1ULL << feat)) {
+    return true;
+  }
+  return false;
+}
+
 }  // namespace ROCKSDB_NAMESPACE

file/prefetch_test.cc

@@ -1304,10 +1304,14 @@ TEST_P(PrefetchTest, DBIterLevelReadAhead) {
 // This test verifies the functionality of ReadOptions.adaptive_readahead when
 // async_io is enabled.
 TEST_P(PrefetchTest, DBIterLevelReadAheadWithAsyncIO) {
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
   const int kNumKeys = 1000;
   // Set options
   std::shared_ptr<MockFS> fs =
-      std::make_shared<MockFS>(env_->GetFileSystem(), false);
+      std::make_shared<MockFS>(FileSystem::Default(), false);
   std::unique_ptr<Env> env(new CompositeEnvWrapper(env_, fs));
 
   bool use_direct_io = std::get<0>(GetParam());
@@ -1341,16 +1345,26 @@ TEST_P(PrefetchTest, DBIterLevelReadAheadWithAsyncIO) {
   }
   MoveFilesToLevel(2);
   int buff_async_prefetch_count = 0;
+  int buff_prefetch_count = 0;
   int readahead_carry_over_count = 0;
   int num_sst_files = NumTableFilesAtLevel(2);
   size_t current_readahead_size = 0;
+  bool read_async_called = false;
 
   // Test - Iterate over the keys sequentially.
   {
+    SyncPoint::GetInstance()->SetCallBack(
+        "FilePrefetchBuffer::Prefetch:Start",
+        [&](void*) { buff_prefetch_count++; });
+
     SyncPoint::GetInstance()->SetCallBack(
         "FilePrefetchBuffer::PrefetchAsyncInternal:Start",
         [&](void*) { buff_async_prefetch_count++; });
 
+    SyncPoint::GetInstance()->SetCallBack(
+        "UpdateResults::io_uring_result",
+        [&](void* /*arg*/) { read_async_called = true; });
+
     // The callback checks, since reads are sequential, readahead_size doesn't
     // start from 8KB when iterator moves to next file and its called
     // num_sst_files-1 times (excluding for first file).
@@ -1393,15 +1407,18 @@ TEST_P(PrefetchTest, DBIterLevelReadAheadWithAsyncIO) {
     } else {
       ASSERT_EQ(readahead_carry_over_count, 0);
     }
-    ASSERT_GT(buff_async_prefetch_count, 0);
 
     // Check stats to make sure async prefetch is done.
     {
       HistogramData async_read_bytes;
       options.statistics->histogramData(ASYNC_READ_BYTES, &async_read_bytes);
-      if (ro.async_io) {
+      // Not all platforms support iouring. In that case, ReadAsync in posix
+      // won't submit async requests.
+      if (read_async_called) {
+        ASSERT_GT(buff_async_prefetch_count, 0);
         ASSERT_GT(async_read_bytes.count, 0);
       } else {
+        ASSERT_GT(buff_prefetch_count, 0);
         ASSERT_EQ(async_read_bytes.count, 0);
       }
     }
@@ -1434,6 +1451,7 @@ TEST_P(PrefetchTest, DBIterAsyncIONoIOUring) {
   Status s = TryReopen(options);
   if (use_direct_io && (s.IsNotSupported() || s.IsInvalidArgument())) {
     // If direct IO is not supported, skip the test
+    enable_io_uring = true;
     return;
   } else {
     ASSERT_OK(s);
@@ -1515,7 +1533,8 @@ class PrefetchTest1 : public DBTestBase,
  public:
   PrefetchTest1() : DBTestBase("prefetch_test1", true) {}
 
-  void SetGenericOptions(Env* env, bool use_direct_io, Options& options) {
+  virtual void SetGenericOptions(Env* env, bool use_direct_io,
+                                 Options& options) {
     options = CurrentOptions();
     options.write_buffer_size = 1024;
     options.create_if_missing = true;
@@ -1769,10 +1788,14 @@ TEST_P(PrefetchTest1, DecreaseReadAheadIfInCache) {
 // This test verifies the basic functionality of seek parallelization for
 // async_io.
 TEST_P(PrefetchTest1, SeekParallelizationTest) {
+  if (mem_env_ || encrypted_env_) {
+    ROCKSDB_GTEST_BYPASS("Test requires non-mem or non-encrypted environment");
+    return;
+  }
   const int kNumKeys = 2000;
   // Set options
-  std::shared_ptr<MockFS> fs =
+  std::shared_ptr<MockFS> fs = std::make_shared<MockFS>(
-      std::make_shared<MockFS>(env_->GetFileSystem(), false);
+      FileSystem::Default(), /*support_prefetch=*/false);
   std::unique_ptr<Env> env(new CompositeEnvWrapper(env_, fs));
 
   Options options;
@@ -1805,10 +1828,19 @@ TEST_P(PrefetchTest1, SeekParallelizationTest) {
   ASSERT_OK(db_->CompactRange(CompactRangeOptions(), &least, &greatest));
 
   int buff_prefetch_count = 0;
+  int buff_prefetch_async_count = 0;
 
   SyncPoint::GetInstance()->SetCallBack(
       "FilePrefetchBuffer::PrefetchAsyncInternal:Start",
-      [&](void*) { buff_prefetch_count++; });
+      [&](void*) { buff_prefetch_async_count++; });
+
+  SyncPoint::GetInstance()->SetCallBack("FilePrefetchBuffer::Prefetch:Start",
+                                        [&](void*) { buff_prefetch_count++; });
+
+  bool read_async_called = false;
+  SyncPoint::GetInstance()->SetCallBack(
+      "UpdateResults::io_uring_result",
+      [&](void* /*arg*/) { read_async_called = true; });
 
   SyncPoint::GetInstance()->EnableProcessing();
   ReadOptions ro;
@@ -1843,17 +1875,17 @@ TEST_P(PrefetchTest1, SeekParallelizationTest) {
     iter->Next();
     ASSERT_TRUE(iter->Valid());
 
-    ASSERT_EQ(buff_prefetch_count, 2);
+    HistogramData async_read_bytes;
-
+    options.statistics->histogramData(ASYNC_READ_BYTES, &async_read_bytes);
-    // Check stats to make sure async prefetch is done.
+    // not all platforms support io_uring. In that case it'll fallback to normal
-    {
+    // prefetching without async_io.
-      HistogramData async_read_bytes;
+    if (read_async_called) {
-      options.statistics->histogramData(ASYNC_READ_BYTES, &async_read_bytes);
+      ASSERT_EQ(buff_prefetch_async_count, 2);
       ASSERT_GT(async_read_bytes.count, 0);
       ASSERT_GT(get_perf_context()->number_async_seek, 0);
+    } else {
+      ASSERT_EQ(buff_prefetch_count, 1);
     }
-
-    buff_prefetch_count = 0;
   }
   Close();
 }

file/random_access_file_reader.cc

@@ -314,14 +314,14 @@ IOStatus RandomAccessFileReader::MultiRead(
       // Align and merge the read requests.
       size_t alignment = file_->GetRequiredBufferAlignment();
       for (size_t i = 0; i < num_reqs; i++) {
-        const auto& r = Align(read_reqs[i], alignment);
+        FSReadRequest r = Align(read_reqs[i], alignment);
         if (i == 0) {
           // head
-          aligned_reqs.push_back(r);
+          aligned_reqs.push_back(std::move(r));
 
         } else if (!TryMerge(&aligned_reqs.back(), r)) {
           // head + n
-          aligned_reqs.push_back(r);
+          aligned_reqs.push_back(std::move(r));
 
         } else {
           // unused

file/random_access_file_reader_test.cc

@@ -95,7 +95,18 @@ TEST_F(RandomAccessFileReaderTest, MultiReadDirectIO) {
       "RandomAccessFileReader::MultiRead:AlignedReqs", [&](void* reqs) {
         // Copy reqs, since it's allocated on stack inside MultiRead, which will
         // be deallocated after MultiRead returns.
-        aligned_reqs = *reinterpret_cast<std::vector<FSReadRequest>*>(reqs);
+        size_t i = 0;
+        aligned_reqs.resize(
+            (*reinterpret_cast<std::vector<FSReadRequest>*>(reqs)).size());
+        for (auto& req :
+             (*reinterpret_cast<std::vector<FSReadRequest>*>(reqs))) {
+          aligned_reqs[i].offset = req.offset;
+          aligned_reqs[i].len = req.len;
+          aligned_reqs[i].result = req.result;
+          aligned_reqs[i].status = req.status;
+          aligned_reqs[i].scratch = req.scratch;
+          i++;
+        }
       });
   ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->EnableProcessing();
 
@@ -136,7 +147,7 @@ TEST_F(RandomAccessFileReaderTest, MultiReadDirectIO) {
     reqs.push_back(std::move(r1));
     AlignedBuf aligned_buf;
     ASSERT_OK(r->MultiRead(IOOptions(), reqs.data(), reqs.size(), &aligned_buf,
-                           Env::IO_TOTAL /* rate_limiter_priority */));
+                           Env::IO_TOTAL /*rate_limiter_priority*/));
 
     AssertResult(content, reqs);
 
@@ -181,7 +192,7 @@ TEST_F(RandomAccessFileReaderTest, MultiReadDirectIO) {
     reqs.push_back(std::move(r2));
     AlignedBuf aligned_buf;
     ASSERT_OK(r->MultiRead(IOOptions(), reqs.data(), reqs.size(), &aligned_buf,
-                           Env::IO_TOTAL /* rate_limiter_priority */));
+                           Env::IO_TOTAL /*rate_limiter_priority*/));
 
     AssertResult(content, reqs);
 
@@ -226,7 +237,7 @@ TEST_F(RandomAccessFileReaderTest, MultiReadDirectIO) {
     reqs.push_back(std::move(r2));
     AlignedBuf aligned_buf;
     ASSERT_OK(r->MultiRead(IOOptions(), reqs.data(), reqs.size(), &aligned_buf,
-                           Env::IO_TOTAL /* rate_limiter_priority */));
+                           Env::IO_TOTAL /*rate_limiter_priority*/));
 
     AssertResult(content, reqs);
 
@@ -263,7 +274,7 @@ TEST_F(RandomAccessFileReaderTest, MultiReadDirectIO) {
     reqs.push_back(std::move(r1));
     AlignedBuf aligned_buf;
     ASSERT_OK(r->MultiRead(IOOptions(), reqs.data(), reqs.size(), &aligned_buf,
-                           Env::IO_TOTAL /* rate_limiter_priority */));
+                           Env::IO_TOTAL /*rate_limiter_priority*/));
 
     AssertResult(content, reqs);
 
@@ -283,7 +294,6 @@ TEST_F(RandomAccessFileReaderTest, MultiReadDirectIO) {
   ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->ClearAllCallBacks();
 }
 
-
 TEST(FSReadRequest, Align) {
   FSReadRequest r;
   r.offset = 2000;

include/rocksdb/file_system.h

@@ -79,6 +79,10 @@ enum class IOType : uint8_t {
   kInvalid,
 };
 
+// enum representing various operations supported by underlying FileSystem.
+// These need to be set in SupportedOps API for RocksDB to use them.
+enum FSSupportedOps { kAsyncIO, kFSBuffer };
+
 // Per-request options that can be passed down to the FileSystem
 // implementation. These are hints and are not necessarily guaranteed to be
 // honored. More hints can be added here in the future to indicate things like
@@ -657,7 +661,6 @@ class FileSystem : public Customizable {
                                const IOOptions& options, bool* is_dir,
                                IODebugContext* /*dgb*/) = 0;
 
-  // EXPERIMENTAL
   // Poll for completion of read IO requests. The Poll() method should call the
   // callback functions to indicate completion of read requests.
   // Underlying FS is required to support Poll API. Poll implementation should
@@ -665,28 +668,33 @@ class FileSystem : public Customizable {
   // after the callback has been called.
   // If Poll returns partial results for any reads, its caller reponsibility to
   // call Read or ReadAsync in order to get the remaining bytes.
-  //
-  // Default implementation is to return IOStatus::OK.
-
   virtual IOStatus Poll(std::vector<void*>& /*io_handles*/,
                         size_t /*min_completions*/) {
     return IOStatus::OK();
   }
 
-  // EXPERIMENTAL
   // Abort the read IO requests submitted asynchronously. Underlying FS is
   // required to support AbortIO API. AbortIO implementation should ensure that
   // the all the read requests related to io_handles should be aborted and
   // it shouldn't call the callback for these io_handles.
-  //
-  // Default implementation is to return IOStatus::OK.
   virtual IOStatus AbortIO(std::vector<void*>& /*io_handles*/) {
     return IOStatus::OK();
   }
 
-  // Indicates to upper layers whether the FileSystem supports/uses async IO
+  // Indicates to upper layers which FileSystem operations mentioned in
-  // or not
+  // FSSupportedOps are supported by underlying FileSystem. Each bit in
-  virtual bool use_async_io() { return true; }
+  // supported_ops argument represent corresponding FSSupportedOps operation.
+  // Foreg:
+  //  If async_io is supported by the underlying FileSystem, then supported_ops
+  //  will have corresponding bit (i.e FSSupportedOps::kAsyncIO) set to 1.
+  //
+  // By default, async_io operation is set and FS should override this API and
+  // set all the operations they support provided in FSSupportedOps (including
+  // async_io).
+  virtual void SupportedOps(int64_t& supported_ops) {
+    supported_ops = 0;
+    supported_ops |= (1 << FSSupportedOps::kAsyncIO);
+  }
 
   // If you're adding methods here, remember to add them to EnvWrapper too.
 
@@ -791,6 +799,42 @@ struct FSReadRequest {
   // Output parameter set by underlying FileSystem that represents status of
   // read request.
   IOStatus status;
+
+  // fs_scratch is a data buffer allocated and provided by underlying FileSystem
+  // to RocksDB during reads, when FS wants to provide its own buffer with data
+  // instead of using RocksDB provided FSReadRequest::scratch.
+  //
+  // FileSystem needs to provide a buffer and custom delete function. The
+  // lifecycle of fs_scratch until data is used by RocksDB. The buffer
+  // should be released by RocksDB using custom delete function provided in
+  // unique_ptr fs_scratch.
+  //
+  // Optimization benefits:
+  // This is helpful in cases where underlying FileSystem has to do additional
+  // copy of data to RocksDB provided buffer which can consume CPU cycles. It
+  // can be optimized by avoiding copying to RocksDB buffer and directly using
+  // FS provided buffer.
+  //
+  // How to enable:
+  // In order to enable this option, FS needs to override SupportedOps() API and
+  // set FSSupportedOps::kFSBuffer in SupportedOps() as:
+  //  {
+  //    supported_ops |= (1 << FSSupportedOps::kFSBuffer);
+  //  }
+  //
+  // Work in progress:
+  // Right now it's only enabled for MultiReads (sync and async
+  // both) with non direct io.
+  // If RocksDB provide its own buffer (scratch) during reads, that's a
+  //  signal for FS to use RocksDB buffer.
+  // If FSSupportedOps::kFSBuffer is enabled and scratch == nullptr,
+  //   then FS have to provide its own buffer in fs_scratch.
+  //
+  // NOTE:
+  // - FSReadRequest::result should point to fs_scratch.
+  // - This is needed only if FSSupportedOps::kFSBuffer support is provided by
+  // underlying FS.
+  std::unique_ptr<void, std::function<void(void*)>> fs_scratch;
 };
 
 // A file abstraction for randomly reading the contents of a file.
@@ -885,7 +929,6 @@ class FSRandomAccessFile {
     return IOStatus::NotSupported("InvalidateCache not supported.");
   }
 
-  // EXPERIMENTAL
   // This API reads the requested data in FSReadRequest asynchronously. This is
   // a asynchronous call, i.e it should return after submitting the request.
   //
@@ -907,6 +950,16 @@ class FSRandomAccessFile {
   // request and result and status fields are output parameter set by underlying
   // FileSystem. The data should always be read into scratch field.
   //
+  // How to enable:
+  // In order to enable ReadAsync, FS needs to override SupportedOps() API and
+  // set FSSupportedOps::kAsyncIO in SupportedOps() as:
+  //  {
+  //    supported_ops |= (1 << FSSupportedOps::kAsyncIO);
+  //  }
+  //
+  // Note: If FS supports ReadAsync API, it should also override Poll and
+  // AbortIO API.
+  //
   // Default implementation is to read the data synchronously.
   virtual IOStatus ReadAsync(
       FSReadRequest& req, const IOOptions& opts,
@@ -1550,7 +1603,9 @@ class FileSystemWrapper : public FileSystem {
     return target_->AbortIO(io_handles);
   }
 
-  virtual bool use_async_io() override { return target_->use_async_io(); }
+  virtual void SupportedOps(int64_t& supported_ops) override {
+    return target_->SupportedOps(supported_ops);
+  }
 
  protected:
   std::shared_ptr<FileSystem> target_;

port/win/env_win.h

@@ -227,7 +227,7 @@ class WinFileSystem : public FileSystem {
       const FileOptions& file_options) const override;
   FileOptions OptimizeForManifestWrite(
       const FileOptions& file_options) const override;
-  bool use_async_io() override { return false; }
+  void SupportedOps(int64_t& supported_ops) override { supported_ops = 0; }
 
  protected:
   static uint64_t FileTimeToUnixTime(const FILETIME& ftTime);

table/block_based/block_based_table_reader.h

@@ -377,7 +377,7 @@ class BlockBasedTable : public TableReader {
       const MultiGetRange* batch,
       const autovector<BlockHandle, MultiGetContext::MAX_BATCH_SIZE>* handles,
       Status* statuses, CachableEntry<Block_kData>* results, char* scratch,
-      const UncompressionDict& uncompression_dict);
+      const UncompressionDict& uncompression_dict, bool use_fs_scratch);
 
   // Get the iterator from the index reader.
   //

table/block_based/block_based_table_reader_sync_and_async.h

@@ -33,7 +33,7 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::RetrieveMultipleBlocks)
 (const ReadOptions& options, const MultiGetRange* batch,
  const autovector<BlockHandle, MultiGetContext::MAX_BATCH_SIZE>* handles,
  Status* statuses, CachableEntry<Block_kData>* results, char* scratch,
- const UncompressionDict& uncompression_dict) const {
+ const UncompressionDict& uncompression_dict, bool use_fs_scratch) const {
   RandomAccessFileReader* file = rep_->file.get();
   const Footer& footer = rep_->footer;
   const ImmutableOptions& ioptions = rep_->ioptions;
@@ -88,7 +88,7 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::RetrieveMultipleBlocks)
     // We don't combine block reads here in direct IO mode, because when doing
     // direct IO read, the block requests will be realigned and merged when
     // necessary.
-    if (use_shared_buffer && !file->use_direct_io() &&
+    if ((use_shared_buffer || use_fs_scratch) && !file->use_direct_io() &&
         prev_end == handle.offset()) {
       req_offset_for_block.emplace_back(prev_len);
       prev_len += BlockSizeWithTrailer(handle);
@@ -99,7 +99,7 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::RetrieveMultipleBlocks)
         FSReadRequest req;
         req.offset = prev_offset;
         req.len = prev_len;
-        if (file->use_direct_io()) {
+        if (file->use_direct_io() || use_fs_scratch) {
           req.scratch = nullptr;
         } else if (use_shared_buffer) {
           req.scratch = scratch + buf_offset;
@@ -107,10 +107,10 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::RetrieveMultipleBlocks)
         } else {
           req.scratch = new char[req.len];
         }
-        read_reqs.emplace_back(req);
+        read_reqs.emplace_back(std::move(req));
       }
 
-      // Step 2, remeber the previous block info
+      // Step 2, remember the previous block info
       prev_offset = handle.offset();
       prev_len = BlockSizeWithTrailer(handle);
       req_offset_for_block.emplace_back(0);
@@ -125,14 +125,14 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::RetrieveMultipleBlocks)
     FSReadRequest req;
     req.offset = prev_offset;
     req.len = prev_len;
-    if (file->use_direct_io()) {
+    if (file->use_direct_io() || use_fs_scratch) {
       req.scratch = nullptr;
     } else if (use_shared_buffer) {
       req.scratch = scratch + buf_offset;
     } else {
       req.scratch = new char[req.len];
     }
-    read_reqs.emplace_back(req);
+    read_reqs.emplace_back(std::move(req));
   }
 
   AlignedBuf direct_io_buf;
@@ -192,7 +192,10 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::RetrieveMultipleBlocks)
 
     BlockContents serialized_block;
     if (s.ok()) {
-      if (!use_shared_buffer) {
+      if (use_fs_scratch) {
+        serialized_block =
+            BlockContents(Slice(req.result.data() + req_offset, handle.size()));
+      } else if (!use_shared_buffer) {
         // We allocated a buffer for this block. Give ownership of it to
         // BlockContents so it can free the memory
         assert(req.result.data() == req.scratch);
@@ -243,7 +246,8 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::RetrieveMultipleBlocks)
       // heap buffer or there is no cache at all.
       CompressionType compression_type =
           GetBlockCompressionType(serialized_block);
-      if (use_shared_buffer && compression_type == kNoCompression) {
+      if ((use_fs_scratch || use_shared_buffer) &&
+          compression_type == kNoCompression) {
         Slice serialized =
             Slice(req.result.data() + req_offset, BlockSizeWithTrailer(handle));
         serialized_block = BlockContents(
@@ -304,6 +308,17 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::RetrieveMultipleBlocks)
     }
     statuses[idx_in_batch] = s;
   }
+
+  if (use_fs_scratch) {
+    // Free the allocated scratch buffer by fs here as read requests might have
+    // been combined into one.
+    for (FSReadRequest& req : read_reqs) {
+      if (req.fs_scratch != nullptr) {
+        req.fs_scratch.reset();
+        req.fs_scratch = nullptr;
+      }
+    }
+  }
 }
 
 using MultiGetRange = MultiGetContext::Range;
@@ -512,11 +527,20 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::MultiGet)
 
       if (total_len) {
         char* scratch = nullptr;
+        bool use_fs_scratch = false;
         const UncompressionDict& dict = uncompression_dict.GetValue()
                                             ? *uncompression_dict.GetValue()
                                             : UncompressionDict::GetEmptyDict();
         assert(uncompression_dict_inited || !rep_->uncompression_dict_reader);
         assert(uncompression_dict_status.ok());
+
+        if (!rep_->file->use_direct_io()) {
+          if (CheckFSFeatureSupport(rep_->ioptions.fs.get(),
+                                    FSSupportedOps::kFSBuffer)) {
+            use_fs_scratch = true;
+          }
+        }
+
         // If using direct IO, then scratch is not used, so keep it nullptr.
         // If the blocks need to be uncompressed and we don't need the
         // compressed blocks, then we can use a contiguous block of
@@ -527,7 +551,7 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::MultiGet)
         // 2. If blocks are uncompressed, alloc heap bufs
         // 3. If blocks are compressed and no compressed block cache, use
         //    stack buf
-        if (!rep_->file->use_direct_io() &&
+        if (!use_fs_scratch && !rep_->file->use_direct_io() &&
             rep_->blocks_maybe_compressed) {
           if (total_len <= kMultiGetReadStackBufSize) {
             scratch = stack_buf;
@@ -538,7 +562,7 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::MultiGet)
         }
         CO_AWAIT(RetrieveMultipleBlocks)
         (read_options, &data_block_range, &block_handles, &statuses[0],
-         &results[0], scratch, dict);
+         &results[0], scratch, dict, use_fs_scratch);
         if (get_context) {
           ++(get_context->get_context_stats_.num_sst_read);
         }

unreleased_history/new_features/fs_provided_buffer.md

@@ -0,0 +1 @@
+Add FSReadRequest::fs_scratch which is a data buffer allocated and provided by underlying FileSystem to RocksDB during reads, when FS wants to provide its own buffer with data instead of using RocksDB provided FSReadRequest::scratch. This can help in cpu optimization by avoiding copy from file system's buffer to RocksDB buffer. More details on how to use/enable it in file_system.h. Right now its supported only for MultiReads(async + sync) with non direct io.

unreleased_history/public_api_changes/change_async_io_api.md

@@ -0,0 +1 @@
+change the FileSystem::use_async_io() API to SupportedOps API in order to extend it to various operations supported by underlying FileSystem. Right now it contains FSSupportedOps::kAsyncIO and FSSupportedOps::kFSBuffer. More details about FSSupportedOps in filesystem.h