Added support for sequential read-ahead file (#5580)

Summary:
Added support for sequential read-ahead file that can prefetch the read data and later serve it from internal cache buffer.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5580

Differential Revision: D16287082

Pulled By: elipoz

fbshipit-source-id: a3e7ad9643d377d39352ff63058ce050ec31dcf3

test_util/testutil.h

@@ -492,13 +492,11 @@ inline std::string EncodeInt(uint64_t x) {
   return result;
 }
 
-class StringEnv : public EnvWrapper {
- public:
   class SeqStringSource : public SequentialFile {
    public:
     explicit SeqStringSource(const std::string& data)
         : data_(data), offset_(0) {}
-    ~SeqStringSource() {}
+    ~SeqStringSource() override {}
     Status Read(size_t n, Slice* result, char* scratch) override {
       std::string output;
       if (offset_ < data_.size()) {
@@ -527,129 +525,136 @@ class StringEnv : public EnvWrapper {
     size_t offset_;
   };
 
-  class StringSink : public WritableFile {
+  class StringEnv : public EnvWrapper {
    public:
-    explicit StringSink(std::string* contents)
+    class StringSink : public WritableFile {
-        : WritableFile(), contents_(contents) {}
+     public:
-    virtual Status Truncate(uint64_t size) override {
+      explicit StringSink(std::string* contents)
-      contents_->resize(static_cast<size_t>(size));
+          : WritableFile(), contents_(contents) {}
-      return Status::OK();
+      virtual Status Truncate(uint64_t size) override {
-    }
+        contents_->resize(static_cast<size_t>(size));
-    virtual Status Close() override { return Status::OK(); }
+        return Status::OK();
-    virtual Status Flush() override { return Status::OK(); }
+      }
-    virtual Status Sync() override { return Status::OK(); }
+      virtual Status Close() override { return Status::OK(); }
-    virtual Status Append(const Slice& slice) override {
+      virtual Status Flush() override { return Status::OK(); }
-      contents_->append(slice.data(), slice.size());
+      virtual Status Sync() override { return Status::OK(); }
+      virtual Status Append(const Slice& slice) override {
+        contents_->append(slice.data(), slice.size());
+        return Status::OK();
+      }
+
+     private:
+      std::string* contents_;
+    };
+
+    explicit StringEnv(Env* t) : EnvWrapper(t) {}
+    ~StringEnv() override {}
+
+    const std::string& GetContent(const std::string& f) { return files_[f]; }
+
+    const Status WriteToNewFile(const std::string& file_name,
+                                const std::string& content) {
+      std::unique_ptr<WritableFile> r;
+      auto s = NewWritableFile(file_name, &r, EnvOptions());
+      if (!s.ok()) {
+        return s;
+      }
+      r->Append(content);
+      r->Flush();
+      r->Close();
+      assert(files_[file_name] == content);
       return Status::OK();
     }
 
-   private:
+    // The following text is boilerplate that forwards all methods to target()
-    std::string* contents_;
+    Status NewSequentialFile(const std::string& f,
-  };
+                             std::unique_ptr<SequentialFile>* r,
-
-  explicit StringEnv(Env* t) : EnvWrapper(t) {}
-  virtual ~StringEnv() {}
-
-  const std::string& GetContent(const std::string& f) { return files_[f]; }
-
-  const Status WriteToNewFile(const std::string& file_name,
-                              const std::string& content) {
-    std::unique_ptr<WritableFile> r;
-    auto s = NewWritableFile(file_name, &r, EnvOptions());
-    if (!s.ok()) {
-      return s;
-    }
-    r->Append(content);
-    r->Flush();
-    r->Close();
-    assert(files_[file_name] == content);
-    return Status::OK();
-  }
-
-  // The following text is boilerplate that forwards all methods to target()
-  Status NewSequentialFile(const std::string& f,
-                           std::unique_ptr<SequentialFile>* r,
-                           const EnvOptions& /*options*/) override {
-    auto iter = files_.find(f);
-    if (iter == files_.end()) {
-      return Status::NotFound("The specified file does not exist", f);
-    }
-    r->reset(new SeqStringSource(iter->second));
-    return Status::OK();
-  }
-  Status NewRandomAccessFile(const std::string& /*f*/,
-                             std::unique_ptr<RandomAccessFile>* /*r*/,
                              const EnvOptions& /*options*/) override {
-    return Status::NotSupported();
+      auto iter = files_.find(f);
-  }
+      if (iter == files_.end()) {
-  Status NewWritableFile(const std::string& f, std::unique_ptr<WritableFile>* r,
+        return Status::NotFound("The specified file does not exist", f);
-                         const EnvOptions& /*options*/) override {
+      }
-    auto iter = files_.find(f);
+      r->reset(new SeqStringSource(iter->second));
-    if (iter != files_.end()) {
+      return Status::OK();
-      return Status::IOError("The specified file already exists", f);
     }
-    r->reset(new StringSink(&files_[f]));
+    Status NewRandomAccessFile(const std::string& /*f*/,
-    return Status::OK();
+                               std::unique_ptr<RandomAccessFile>* /*r*/,
-  }
+                               const EnvOptions& /*options*/) override {
-  virtual Status NewDirectory(const std::string& /*name*/,
+      return Status::NotSupported();
-                              std::unique_ptr<Directory>* /*result*/) override {
-    return Status::NotSupported();
-  }
-  Status FileExists(const std::string& f) override {
-    if (files_.find(f) == files_.end()) {
-      return Status::NotFound();
     }
-    return Status::OK();
+    Status NewWritableFile(const std::string& f,
-  }
+                           std::unique_ptr<WritableFile>* r,
-  Status GetChildren(const std::string& /*dir*/,
+                           const EnvOptions& /*options*/) override {
-                     std::vector<std::string>* /*r*/) override {
+      auto iter = files_.find(f);
-    return Status::NotSupported();
+      if (iter != files_.end()) {
-  }
+        return Status::IOError("The specified file already exists", f);
-  Status DeleteFile(const std::string& f) override {
+      }
-    files_.erase(f);
+      r->reset(new StringSink(&files_[f]));
-    return Status::OK();
+      return Status::OK();
-  }
+    }
-  Status CreateDir(const std::string& /*d*/) override {
+    virtual Status NewDirectory(
-    return Status::NotSupported();
+        const std::string& /*name*/,
-  }
+        std::unique_ptr<Directory>* /*result*/) override {
-  Status CreateDirIfMissing(const std::string& /*d*/) override {
+      return Status::NotSupported();
-    return Status::NotSupported();
+    }
-  }
+    Status FileExists(const std::string& f) override {
-  Status DeleteDir(const std::string& /*d*/) override {
+      if (files_.find(f) == files_.end()) {
-    return Status::NotSupported();
+        return Status::NotFound();
-  }
+      }
-  Status GetFileSize(const std::string& f, uint64_t* s) override {
+      return Status::OK();
-    auto iter = files_.find(f);
+    }
-    if (iter == files_.end()) {
+    Status GetChildren(const std::string& /*dir*/,
-      return Status::NotFound("The specified file does not exist:", f);
+                       std::vector<std::string>* /*r*/) override {
+      return Status::NotSupported();
+    }
+    Status DeleteFile(const std::string& f) override {
+      files_.erase(f);
+      return Status::OK();
+    }
+    Status CreateDir(const std::string& /*d*/) override {
+      return Status::NotSupported();
+    }
+    Status CreateDirIfMissing(const std::string& /*d*/) override {
+      return Status::NotSupported();
+    }
+    Status DeleteDir(const std::string& /*d*/) override {
+      return Status::NotSupported();
+    }
+    Status GetFileSize(const std::string& f, uint64_t* s) override {
+      auto iter = files_.find(f);
+      if (iter == files_.end()) {
+        return Status::NotFound("The specified file does not exist:", f);
+      }
+      *s = iter->second.size();
+      return Status::OK();
     }
-    *s = iter->second.size();
-    return Status::OK();
-  }
 
-  Status GetFileModificationTime(const std::string& /*fname*/,
+    Status GetFileModificationTime(const std::string& /*fname*/,
-                                 uint64_t* /*file_mtime*/) override {
+                                   uint64_t* /*file_mtime*/) override {
-    return Status::NotSupported();
+      return Status::NotSupported();
-  }
+    }
 
-  Status RenameFile(const std::string& /*s*/,
+    Status RenameFile(const std::string& /*s*/,
+                      const std::string& /*t*/) override {
+      return Status::NotSupported();
+    }
+
+    Status LinkFile(const std::string& /*s*/,
                     const std::string& /*t*/) override {
-    return Status::NotSupported();
+      return Status::NotSupported();
-  }
+    }
 
-  Status LinkFile(const std::string& /*s*/, const std::string& /*t*/) override {
+    Status LockFile(const std::string& /*f*/, FileLock** /*l*/) override {
-    return Status::NotSupported();
+      return Status::NotSupported();
-  }
+    }
 
-  Status LockFile(const std::string& /*f*/, FileLock** /*l*/) override {
+    Status UnlockFile(FileLock* /*l*/) override {
-    return Status::NotSupported();
+      return Status::NotSupported();
-  }
+    }
 
-  Status UnlockFile(FileLock* /*l*/) override { return Status::NotSupported(); }
+   protected:
-
+    std::unordered_map<std::string, std::string> files_;
- protected:
+  };
-  std::unordered_map<std::string, std::string> files_;
-};
 
 // Randomly initialize the given DBOptions
 void RandomInitDBOptions(DBOptions* db_opt, Random* rnd);

util/file_reader_writer.cc

@@ -639,6 +639,7 @@ class ReadaheadRandomAccessFile : public RandomAccessFile {
 
  Status Read(uint64_t offset, size_t n, Slice* result,
              char* scratch) const override {
+   // Read-ahead only make sense if we have some slack left after reading
    if (n + alignment_ >= readahead_size_) {
      return file_->Read(offset, n, result, scratch);
    }
@@ -646,14 +647,13 @@ class ReadaheadRandomAccessFile : public RandomAccessFile {
    std::unique_lock<std::mutex> lk(lock_);
 
    size_t cached_len = 0;
-   // Check if there is a cache hit, means that [offset, offset + n) is either
+   // Check if there is a cache hit, meaning that [offset, offset + n) is either
-   // completely or partially in the buffer
+   // completely or partially in the buffer.
    // If it's completely cached, including end of file case when offset + n is
-   // greater than EOF, return
+   // greater than EOF, then return.
    if (TryReadFromCache(offset, n, &cached_len, scratch) &&
-       (cached_len == n ||
+       (cached_len == n || buffer_.CurrentSize() < readahead_size_)) {
-        // End of file
+     // We read exactly what we needed, or we hit end of file - return.
-        buffer_.CurrentSize() < readahead_size_)) {
      *result = Slice(scratch, cached_len);
      return Status::OK();
    }
@@ -661,25 +661,14 @@ class ReadaheadRandomAccessFile : public RandomAccessFile {
    // In the case of cache hit advanced_offset is already aligned, means that
    // chunk_offset equals to advanced_offset
    size_t chunk_offset = TruncateToPageBoundary(alignment_, advanced_offset);
-   Slice readahead_result;
 
    Status s = ReadIntoBuffer(chunk_offset, readahead_size_);
    if (s.ok()) {
-     // In the case of cache miss, i.e. when cached_len equals 0, an offset can
+     // The data we need is now in cache, so we can safely read it
-     // exceed the file end position, so the following check is required
+     size_t remaining_len;
-     if (advanced_offset < chunk_offset + buffer_.CurrentSize()) {
+     TryReadFromCache(advanced_offset, n - cached_len, &remaining_len,
-       // In the case of cache miss, the first chunk_padding bytes in buffer_
+                      scratch + cached_len);
-       // are
+     *result = Slice(scratch, cached_len + remaining_len);
-       // stored for alignment only and must be skipped
-       size_t chunk_padding = advanced_offset - chunk_offset;
-       auto remaining_len =
-           std::min(buffer_.CurrentSize() - chunk_padding, n - cached_len);
-       memcpy(scratch + cached_len, buffer_.BufferStart() + chunk_padding,
-              remaining_len);
-       *result = Slice(scratch, cached_len + remaining_len);
-     } else {
-       *result = Slice(scratch, cached_len);
-     }
    }
    return s;
  }
@@ -690,6 +679,9 @@ class ReadaheadRandomAccessFile : public RandomAccessFile {
      // `Read()` assumes a smaller prefetch buffer indicates EOF was reached.
      return Status::OK();
    }
+
+   std::unique_lock<std::mutex> lk(lock_);
+
    size_t offset_ = static_cast<size_t>(offset);
    size_t prefetch_offset = TruncateToPageBoundary(alignment_, offset_);
    if (prefetch_offset == buffer_offset_) {
@@ -706,12 +698,18 @@ class ReadaheadRandomAccessFile : public RandomAccessFile {
  void Hint(AccessPattern pattern) override { file_->Hint(pattern); }
 
  Status InvalidateCache(size_t offset, size_t length) override {
+   std::unique_lock<std::mutex> lk(lock_);
+   buffer_.Clear();
    return file_->InvalidateCache(offset, length);
  }
 
  bool use_direct_io() const override { return file_->use_direct_io(); }
 
 private:
+ // Tries to read from buffer_ n bytes starting at offset. If anything was read
+ // from the cache, it sets cached_len to the number of bytes actually read,
+ // copies these number of bytes to scratch and returns true.
+ // If nothing was read sets cached_len to 0 and returns false.
  bool TryReadFromCache(uint64_t offset, size_t n, size_t* cached_len,
                        char* scratch) const {
    if (offset < buffer_offset_ ||
@@ -726,6 +724,9 @@ private:
    return true;
   }
 
+  // Reads into buffer_ the next n bytes from file_ starting at offset.
+  // Can actually read less if EOF was reached.
+  // Returns the status of the read operastion on the file.
   Status ReadIntoBuffer(uint64_t offset, size_t n) const {
     if (n > buffer_.Capacity()) {
       n = buffer_.Capacity();
@@ -742,14 +743,171 @@ private:
     return s;
   }
 
-  std::unique_ptr<RandomAccessFile> file_;
+  const std::unique_ptr<RandomAccessFile> file_;
   const size_t alignment_;
-  size_t               readahead_size_;
+  const size_t readahead_size_;
 
   mutable std::mutex lock_;
+  // The buffer storing the prefetched data
   mutable AlignedBuffer buffer_;
+  // The offset in file_, corresponding to data stored in buffer_
   mutable uint64_t buffer_offset_;
 };
+
+// This class wraps a SequentialFile, exposing same API, with the differenece
+// of being able to prefetch up to readahead_size bytes and then serve them
+// from memory, avoiding the entire round-trip if, for example, the data for the
+// file is actually remote.
+class ReadaheadSequentialFile : public SequentialFile {
+ public:
+  ReadaheadSequentialFile(std::unique_ptr<SequentialFile>&& file,
+                          size_t readahead_size)
+      : file_(std::move(file)),
+        alignment_(file_->GetRequiredBufferAlignment()),
+        readahead_size_(Roundup(readahead_size, alignment_)),
+        buffer_(),
+        buffer_offset_(0),
+        read_offset_(0) {
+    buffer_.Alignment(alignment_);
+    buffer_.AllocateNewBuffer(readahead_size_);
+  }
+
+  ReadaheadSequentialFile(const ReadaheadSequentialFile&) = delete;
+
+  ReadaheadSequentialFile& operator=(const ReadaheadSequentialFile&) = delete;
+
+  Status Read(size_t n, Slice* result, char* scratch) override {
+    std::unique_lock<std::mutex> lk(lock_);
+
+    size_t cached_len = 0;
+    // Check if there is a cache hit, meaning that [offset, offset + n) is
+    // either completely or partially in the buffer. If it's completely cached,
+    // including end of file case when offset + n is greater than EOF, then
+    // return.
+    if (TryReadFromCache(n, &cached_len, scratch) &&
+        (cached_len == n || buffer_.CurrentSize() < readahead_size_)) {
+      // We read exactly what we needed, or we hit end of file - return.
+      *result = Slice(scratch, cached_len);
+      return Status::OK();
+    }
+    n -= cached_len;
+
+    Status s;
+    // Read-ahead only make sense if we have some slack left after reading
+    if (n + alignment_ >= readahead_size_) {
+      s = file_->Read(n, result, scratch + cached_len);
+      if (s.ok()) {
+        read_offset_ += result->size();
+        *result = Slice(scratch, cached_len + result->size());
+      }
+      buffer_.Clear();
+      return s;
+    }
+
+    s = ReadIntoBuffer(readahead_size_);
+    if (s.ok()) {
+      // The data we need is now in cache, so we can safely read it
+      size_t remaining_len;
+      TryReadFromCache(n, &remaining_len, scratch + cached_len);
+      *result = Slice(scratch, cached_len + remaining_len);
+    }
+    return s;
+  }
+
+  Status Skip(uint64_t n) override {
+    std::unique_lock<std::mutex> lk(lock_);
+    Status s = Status::OK();
+    // First check if we need to skip already cached data
+    if (buffer_.CurrentSize() > 0) {
+      // Do we need to skip beyond cached data?
+      if (read_offset_ + n >= buffer_offset_ + buffer_.CurrentSize()) {
+        // Yes. Skip whaterver is in memory and adjust offset accordingly
+        n -= buffer_offset_ + buffer_.CurrentSize() - read_offset_;
+        read_offset_ = buffer_offset_ + buffer_.CurrentSize();
+      } else {
+        // No. The entire section to be skipped is entirely i cache.
+        read_offset_ += n;
+        n = 0;
+      }
+    }
+    if (n > 0) {
+      // We still need to skip more, so call the file API for skipping
+      s = file_->Skip(n);
+      if (s.ok()) {
+        read_offset_ += n;
+      }
+      buffer_.Clear();
+    }
+    return s;
+  }
+
+  Status PositionedRead(uint64_t offset, size_t n, Slice* result,
+                        char* scratch) override {
+    return file_->PositionedRead(offset, n, result, scratch);
+  }
+
+  Status InvalidateCache(size_t offset, size_t length) override {
+    std::unique_lock<std::mutex> lk(lock_);
+    buffer_.Clear();
+    return file_->InvalidateCache(offset, length);
+  }
+
+  bool use_direct_io() const override { return file_->use_direct_io(); }
+
+ private:
+  // Tries to read from buffer_ n bytes. If anything was read from the cache, it
+  // sets cached_len to the number of bytes actually read, copies these number
+  // of bytes to scratch and returns true.
+  // If nothing was read sets cached_len to 0 and returns false.
+  bool TryReadFromCache(size_t n, size_t* cached_len, char* scratch) {
+    if (read_offset_ < buffer_offset_ ||
+        read_offset_ >= buffer_offset_ + buffer_.CurrentSize()) {
+      *cached_len = 0;
+      return false;
+    }
+    uint64_t offset_in_buffer = read_offset_ - buffer_offset_;
+    *cached_len = std::min(
+        buffer_.CurrentSize() - static_cast<size_t>(offset_in_buffer), n);
+    memcpy(scratch, buffer_.BufferStart() + offset_in_buffer, *cached_len);
+    read_offset_ += *cached_len;
+    return true;
+  }
+
+  // Reads into buffer_ the next n bytes from file_.
+  // Can actually read less if EOF was reached.
+  // Returns the status of the read operastion on the file.
+  Status ReadIntoBuffer(size_t n) {
+    if (n > buffer_.Capacity()) {
+      n = buffer_.Capacity();
+    }
+    assert(IsFileSectorAligned(n, alignment_));
+    Slice result;
+    Status s = file_->Read(n, &result, buffer_.BufferStart());
+    if (s.ok()) {
+      buffer_offset_ = read_offset_;
+      buffer_.Size(result.size());
+      assert(buffer_.BufferStart() == result.data());
+    }
+    return s;
+  }
+
+  const std::unique_ptr<SequentialFile> file_;
+  const size_t alignment_;
+  const size_t readahead_size_;
+
+  std::mutex lock_;
+  // The buffer storing the prefetched data
+  AlignedBuffer buffer_;
+  // The offset in file_, corresponding to data stored in buffer_
+  uint64_t buffer_offset_;
+  // The offset up to which data was read from file_. In fact, it can be larger
+  // than the actual file size, since the file_->Skip(n) call doesn't return the
+  // actual number of bytes that were skipped, which can be less than n.
+  // This is not a problemm since read_offset_ is monotonically increasing and
+  // its only use is to figure out if next piece of data should be read from
+  // buffer_ or file_ directly.
+  uint64_t read_offset_;
+};
 }  // namespace
 
 Status FilePrefetchBuffer::Prefetch(RandomAccessFileReader* reader,
@@ -866,6 +1024,14 @@ std::unique_ptr<RandomAccessFile> NewReadaheadRandomAccessFile(
   return result;
 }
 
+std::unique_ptr<SequentialFile>
+SequentialFileReader::NewReadaheadSequentialFile(
+    std::unique_ptr<SequentialFile>&& file, size_t readahead_size) {
+  std::unique_ptr<SequentialFile> result(
+      new ReadaheadSequentialFile(std::move(file), readahead_size));
+  return result;
+}
+
 Status NewWritableFile(Env* env, const std::string& fname,
                        std::unique_ptr<WritableFile>* result,
                        const EnvOptions& options) {

util/file_reader_writer.h

@@ -43,12 +43,18 @@ class SequentialFileReader {
  private:
   std::unique_ptr<SequentialFile> file_;
   std::string file_name_;
-  std::atomic<size_t> offset_;  // read offset
+  std::atomic<size_t> offset_{0};  // read offset
 
  public:
   explicit SequentialFileReader(std::unique_ptr<SequentialFile>&& _file,
                                 const std::string& _file_name)
-      : file_(std::move(_file)), file_name_(_file_name), offset_(0) {}
+      : file_(std::move(_file)), file_name_(_file_name) {}
+
+  explicit SequentialFileReader(std::unique_ptr<SequentialFile>&& _file,
+                                const std::string& _file_name,
+                                size_t _readahead_size)
+      : file_(NewReadaheadSequentialFile(std::move(_file), _readahead_size)),
+        file_name_(_file_name) {}
 
   SequentialFileReader(SequentialFileReader&& o) ROCKSDB_NOEXCEPT {
     *this = std::move(o);
@@ -66,13 +72,17 @@ class SequentialFileReader {
 
   Status Skip(uint64_t n);
 
-  void Rewind();
-
   SequentialFile* file() { return file_.get(); }
 
   std::string file_name() { return file_name_; }
 
   bool use_direct_io() const { return file_->use_direct_io(); }
+
+ private:
+  // NewReadaheadSequentialFile provides a wrapper over SequentialFile to
+  // always prefetch additional data with every read.
+  static std::unique_ptr<SequentialFile> NewReadaheadSequentialFile(
+      std::unique_ptr<SequentialFile>&& file, size_t readahead_size);
 };
 
 // RandomAccessFileReader is a wrapper on top of Env::RnadomAccessFile. It is

util/file_reader_writer_test.cc

@@ -275,7 +275,7 @@ TEST_P(ReadaheadRandomAccessFileTest, SourceStrLenLessThanReadaheadSizeTest) {
 }
 
 TEST_P(ReadaheadRandomAccessFileTest,
-       SourceStrLenCanBeGreaterThanReadaheadSizeTest) {
+       SourceStrLenGreaterThanReadaheadSizeTest) {
   Random rng(42);
   for (int k = 0; k < 100; ++k) {
     size_t strLen = k * GetReadaheadSize() +
@@ -286,13 +286,13 @@ TEST_P(ReadaheadRandomAccessFileTest,
     for (int test = 1; test <= 100; ++test) {
       size_t offset = rng.Uniform(static_cast<int>(strLen));
       size_t n = rng.Uniform(static_cast<int>(GetReadaheadSize()));
-      ASSERT_EQ(str.substr(offset, std::min(n, str.size() - offset)),
+      ASSERT_EQ(str.substr(offset, std::min(n, strLen - offset)),
                 Read(offset, n));
     }
   }
 }
 
-TEST_P(ReadaheadRandomAccessFileTest, NExceedReadaheadTest) {
+TEST_P(ReadaheadRandomAccessFileTest, ReadExceedsReadaheadSizeTest) {
   Random rng(7);
   size_t strLen = 4 * GetReadaheadSize() +
                   rng.Uniform(static_cast<int>(GetReadaheadSize()));
@@ -303,7 +303,7 @@ TEST_P(ReadaheadRandomAccessFileTest, NExceedReadaheadTest) {
     size_t offset = rng.Uniform(static_cast<int>(strLen));
     size_t n =
         GetReadaheadSize() + rng.Uniform(static_cast<int>(GetReadaheadSize()));
-    ASSERT_EQ(str.substr(offset, std::min(n, str.size() - offset)),
+    ASSERT_EQ(str.substr(offset, std::min(n, strLen - offset)),
               Read(offset, n));
   }
 }
@@ -315,13 +315,118 @@ INSTANTIATE_TEST_CASE_P(
     SourceStrLenLessThanReadaheadSizeTest, ReadaheadRandomAccessFileTest,
     ::testing::ValuesIn(ReadaheadRandomAccessFileTest::GetReadaheadSizeList()));
 INSTANTIATE_TEST_CASE_P(
-    SourceStrLenCanBeGreaterThanReadaheadSizeTest,
+    SourceStrLenGreaterThanReadaheadSizeTest, ReadaheadRandomAccessFileTest,
-    ReadaheadRandomAccessFileTest,
     ::testing::ValuesIn(ReadaheadRandomAccessFileTest::GetReadaheadSizeList()));
 INSTANTIATE_TEST_CASE_P(
-    NExceedReadaheadTest, ReadaheadRandomAccessFileTest,
+    ReadExceedsReadaheadSizeTest, ReadaheadRandomAccessFileTest,
     ::testing::ValuesIn(ReadaheadRandomAccessFileTest::GetReadaheadSizeList()));
 
+class ReadaheadSequentialFileTest : public testing::Test,
+                                    public testing::WithParamInterface<size_t> {
+ public:
+  static std::vector<size_t> GetReadaheadSizeList() {
+    return {1lu << 12, 1lu << 16};
+  }
+  void SetUp() override {
+    readahead_size_ = GetParam();
+    scratch_.reset(new char[2 * readahead_size_]);
+    ResetSourceStr();
+  }
+  ReadaheadSequentialFileTest() {}
+  std::string Read(size_t n) {
+    Slice result;
+    test_read_holder_->Read(n, &result, scratch_.get());
+    return std::string(result.data(), result.size());
+  }
+  void Skip(size_t n) { test_read_holder_->Skip(n); }
+  void ResetSourceStr(const std::string& str = "") {
+    auto read_holder =
+        std::unique_ptr<SequentialFile>(new test::SeqStringSource(str));
+    test_read_holder_.reset(new SequentialFileReader(std::move(read_holder),
+                                                     "test", readahead_size_));
+  }
+  size_t GetReadaheadSize() const { return readahead_size_; }
+
+ private:
+  size_t readahead_size_;
+  std::unique_ptr<SequentialFileReader> test_read_holder_;
+  std::unique_ptr<char[]> scratch_;
+};
+
+TEST_P(ReadaheadSequentialFileTest, EmptySourceStrTest) {
+  ASSERT_EQ("", Read(0));
+  ASSERT_EQ("", Read(1));
+  ASSERT_EQ("", Read(13));
+}
+
+TEST_P(ReadaheadSequentialFileTest, SourceStrLenLessThanReadaheadSizeTest) {
+  std::string str = "abcdefghijklmnopqrs";
+  ResetSourceStr(str);
+  ASSERT_EQ(str.substr(0, 3), Read(3));
+  ASSERT_EQ(str.substr(3, 1), Read(1));
+  ASSERT_EQ(str.substr(4), Read(str.size()));
+  ASSERT_EQ("", Read(100));
+}
+
+TEST_P(ReadaheadSequentialFileTest, SourceStrLenGreaterThanReadaheadSizeTest) {
+  Random rng(42);
+  for (int s = 0; s < 1; ++s) {
+    for (int k = 0; k < 100; ++k) {
+      size_t strLen = k * GetReadaheadSize() +
+                      rng.Uniform(static_cast<int>(GetReadaheadSize()));
+      std::string str =
+          test::RandomHumanReadableString(&rng, static_cast<int>(strLen));
+      ResetSourceStr(str);
+      size_t offset = 0;
+      for (int test = 1; test <= 100; ++test) {
+        size_t n = rng.Uniform(static_cast<int>(GetReadaheadSize()));
+        if (s && test % 2) {
+          Skip(n);
+        } else {
+          ASSERT_EQ(str.substr(offset, std::min(n, strLen - offset)), Read(n));
+        }
+        offset = std::min(offset + n, strLen);
+      }
+    }
+  }
+}
+
+TEST_P(ReadaheadSequentialFileTest, ReadExceedsReadaheadSizeTest) {
+  Random rng(42);
+  for (int s = 0; s < 1; ++s) {
+    for (int k = 0; k < 100; ++k) {
+      size_t strLen = k * GetReadaheadSize() +
+                      rng.Uniform(static_cast<int>(GetReadaheadSize()));
+      std::string str =
+          test::RandomHumanReadableString(&rng, static_cast<int>(strLen));
+      ResetSourceStr(str);
+      size_t offset = 0;
+      for (int test = 1; test <= 100; ++test) {
+        size_t n = GetReadaheadSize() +
+                   rng.Uniform(static_cast<int>(GetReadaheadSize()));
+        if (s && test % 2) {
+          Skip(n);
+        } else {
+          ASSERT_EQ(str.substr(offset, std::min(n, strLen - offset)), Read(n));
+        }
+        offset = std::min(offset + n, strLen);
+      }
+    }
+  }
+}
+
+INSTANTIATE_TEST_CASE_P(
+    EmptySourceStrTest, ReadaheadSequentialFileTest,
+    ::testing::ValuesIn(ReadaheadSequentialFileTest::GetReadaheadSizeList()));
+INSTANTIATE_TEST_CASE_P(
+    SourceStrLenLessThanReadaheadSizeTest, ReadaheadSequentialFileTest,
+    ::testing::ValuesIn(ReadaheadSequentialFileTest::GetReadaheadSizeList()));
+INSTANTIATE_TEST_CASE_P(
+    SourceStrLenGreaterThanReadaheadSizeTest, ReadaheadSequentialFileTest,
+    ::testing::ValuesIn(ReadaheadSequentialFileTest::GetReadaheadSizeList()));
+INSTANTIATE_TEST_CASE_P(
+    ReadExceedsReadaheadSizeTest, ReadaheadSequentialFileTest,
+    ::testing::ValuesIn(ReadaheadSequentialFileTest::GetReadaheadSizeList()));
 }  // namespace rocksdb
 
 int main(int argc, char** argv) {