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Summary: Pull Request resolved: https://github.com/meta/rocksdb/pull/15493 X-link: https://github.com/facebook/rocksdb/pull/15493 **What:** with `ReadOptions::async_io`, MultiGet now reads direct-IO files asynchronously too. Before, `RetrieveMultipleBlocks` sent them to the synchronous `MultiRead`, so a MultiGet over direct-IO files got no read overlap at all. Async reads are now also charged to the rate limiter, like sync reads. **How:** - `AsyncFileReader::MultiReadAsyncImpl`: for a direct-IO request without a scratch buffer, `RandomAccessFileReader::ReadAsync` allocates the aligned buffer and hands it back in the request's `fs_scratch`, with `result` pointing into it. That uses the existing `aligned_buf` support in `ReadAsync`, so the file system only sees aligned requests. If `ReadAsync` reports NotSupported, the synchronous fallback reads through an `AlignedBufferAllocationContext` and moves that buffer into `fs_scratch` the same way. - `RandomAccessFileReader::ReadAsync` charges the read rate limiter at `rate_limiter_priority` for every read it submits: the aligned length for direct IO, the request length otherwise. It charges once the file system accepts the read, because every caller falls back to `Read` on NotSupported and `Read` charges for itself. `MultiRead` and `ReadAsync` share the charging loop (`ChargeRateLimiterForRead`). - `RetrieveMultipleBlocks`: direct-IO files now take `MultiReadAsync` like buffered files. The requests (and so their `fs_scratch`) live until the blocks are parsed. **Behavior changes:** - Direct IO: each block request is aligned and read on its own; requests aren't merged. Adjacent blocks of one file can read an overlapping aligned page twice. - Async reads (async_io MultiGet and iterator prefetching), buffered or direct, are now charged to `DBOptions::rate_limiter`. Before, `ReadAsync` charged nothing. Reviewed By: archang19, anand1976 Differential Revision: D127688099 fbshipit-source-id: cfab475b6a27cb2a78b610144f9f8436c3b8a807