[RocksDB] [Performance] Allow different posix advice to be applied to the same table file

Summary:
Current posix advice implementation ties up the access pattern hint with the creation of a file.
It is not possible to apply different advice for different access (random get vs compaction read),
without keeping two open files for the same table. This patch extended the RandomeAccessFile interface
to accept new access hint at anytime. Particularly, we are able to set different access hint on the same
table file based on when/how the file is used.
Two options are added to set the access hint, after the file is first opened and after the file is being
compacted.

Test Plan: make check; db_stress; db_bench

Reviewers: dhruba

Reviewed By: dhruba

CC: MarkCallaghan, leveldb

Differential Revision: https://reviews.facebook.net/D10905

include/leveldb/env.h

@@ -249,6 +249,12 @@ class RandomAccessFile {
     return 0; // Default implementation to prevent issues with backwards
               // compatibility.
   };
+
+
+  enum AccessPattern { NORMAL, RANDOM, SEQUENTIAL, WILLNEED, DONTNEED };
+
+  virtual void Hint(AccessPattern pattern) {}
+
 };
 
 // A file abstraction for sequential writing.  The implementation

include/leveldb/options.h

@@ -441,6 +441,16 @@ struct Options {
   // new record will be written to the next block.
   // Default is 10.
   int block_size_deviation;
+
+  // If set true, will hint the underlying file system that the file
+  // access pattern is random, when a sst file is opened.
+  // Default: true
+  bool advise_random_on_open;
+
+  // Specify the file access pattern once a compaction is started.
+  // It will be applied to all input files of a compaction.
+  // Default: NORMAL
+  enum { NONE, NORMAL, SEQUENTIAL, WILLNEED } access_hint_on_compaction_start;
 };
 
 // Options that control read operations