sanitize readahead when direct read enabled

Summary:
no readahead:
readseq      :       8.438 micros/op 118510 ops/sec;   13.1 MB/s
sanitize to 10MB:
readseq      :       6.051 micros/op 165248 ops/sec;   18.3 MB/s
Closes https://github.com/facebook/rocksdb/pull/1945

Differential Revision: D4645811

Pulled By: lightmark

fbshipit-source-id: 5d63770

db/db_impl.cc

@@ -197,6 +197,10 @@ DBOptions SanitizeOptions(const std::string& dbname, const DBOptions& src) {
     result.db_paths.emplace_back(dbname, std::numeric_limits<uint64_t>::max());
   }
 
+  if (result.use_direct_reads && result.compaction_readahead_size == 0) {
+    result.compaction_readahead_size = 1024 * 1024 * 2;
+  }
+
   if (result.compaction_readahead_size > 0) {
     result.new_table_reader_for_compaction_inputs = true;
   }