• Cheng Chang's avatar
    Support direct IO in RandomAccessFileReader::MultiRead (#6446) · 4fc21664
    Cheng Chang authored
    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
    4fc21664