Apply use_direct_io_for_flush_and_compaction to writes only
Summary: Previously `DBOptions::use_direct_io_for_flush_and_compaction=true` combined with `DBOptions::use_direct_reads=false` could cause RocksDB to simultaneously read from two file descriptors for the same file, where background reads used direct I/O and foreground reads used buffered I/O. Our measurements found this mixed-mode I/O negatively impacted foreground read perf, compared to when only buffered I/O was used. This PR makes the mixed-mode I/O situation impossible by repurposing `DBOptions::use_direct_io_for_flush_and_compaction` to only apply to background writes, and `DBOptions::use_direct_reads` to apply to all reads. There is no risk of direct background direct writes happening simultaneously with buffered reads since we never read from and write to the same file simultaneously. Closes https://github.com/facebook/rocksdb/pull/3829 Differential Revision: D7915443 Pulled By: ajkr fbshipit-source-id: 78bcbf276449b7e7766ab6b0db246f789fb1b279
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