Support direct IO in RandomAccessFileReader::MultiRead (#6446)

Summary:
By supporting direct IO in RandomAccessFileReader::MultiRead, the benefits of parallel IO (IO uring) and direct IO can be combined.

In direct IO mode, read requests are aligned and merged together before being issued to RandomAccessFile::MultiRead, so blocks in the original requests might share the same underlying buffer, the shared buffers are returned in `aligned_bufs`, which is a new parameter of the `MultiRead` API.

For example, suppose alignment requirement for direct IO is 4KB, one request is (offset: 1KB, len: 1KB), another request is (offset: 3KB, len: 1KB), then since they all belong to page (offset: 0, len: 4KB), `MultiRead` only reads the page with direct IO into a buffer on heap, and returns 2 Slices referencing regions in that same buffer. See `random_access_file_reader_test.cc` for more examples.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6446

Test Plan: Added a new test `random_access_file_reader_test.cc`.

Reviewed By: anand1976

Differential Revision: D20097518

Pulled By: cheng-chang

fbshipit-source-id: ca48a8faf9c3af146465c102ef6b266a363e78d1

1 files changed, 1 insertions, 0 deletions

diff --git a/src.mk b/src.mk
index 628848e73..89558cf96 100644
--- a/src.mk
+++ b/src.mk
@@ -395,6 +395,7 @@ MAIN_SOURCES =                                                          \
   env/env_test.cc                                                       \
   env/io_posix_test.cc                                                  \
   env/mock_env_test.cc                                                  \
+  file/random_access_file_reader_test.cc                                \
   logging/auto_roll_logger_test.cc                                      \
   logging/env_logger_test.cc                                            \
   logging/event_logger_test.cc                                          \