Support Multiple DB paths (without having an interface to expose to users)

Summary:
In this patch, we allow RocksDB to support multiple DB paths internally.
No user interface is supported yet so this patch is silent to users.

Test Plan: make all check

Reviewers: igor, haobo, ljin, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D18921

include/rocksdb/db.h

@@ -55,6 +55,7 @@ class Env;
 // Metadata associated with each SST file.
 struct LiveFileMetaData {
   std::string column_family_name;  // Name of the column family
+  std::string db_path;
   std::string name;                // Name of the file
   int level;               // Level at which this file resides.
   size_t size;             // File size in bytes.

include/rocksdb/options.h

@@ -675,6 +675,13 @@ struct DBOptions {
   // Default value is 1800 (half an hour).
   int db_stats_log_interval;
 
+  // A list paths where SST files can be put into. A compaction style can
+  // determine which of those paths it will put the file to.
+  // If left empty, only one path will be used, which is db_name passed when
+  // opening the DB.
+  // Default: empty
+  std::vector<std::string> db_paths;
+
   // This specifies the info LOG dir.
   // If it is empty, the log files will be in the same dir as data.
   // If it is non empty, the log files will be in the specified dir,

include/rocksdb/universal_compaction.h

@@ -8,6 +8,7 @@
 
 #include <stdint.h>
 #include <climits>
+#include <vector>
 
 namespace rocksdb {
 
@@ -61,6 +62,7 @@ class CompactionOptionsUniversal {
   // well as  the total size of C1...Ct as total_C, the compaction output file
   // will be compressed iff
   //   total_C / total_size < this percentage
+  // Default: -1
   int compression_size_percent;
 
   // The algorithm used to stop picking files into a single compaction run
@@ -68,14 +70,13 @@ class CompactionOptionsUniversal {
   CompactionStopStyle stop_style;
 
   // Default set of parameters
-  CompactionOptionsUniversal() :
-    size_ratio(1),
-    min_merge_width(2),
-    max_merge_width(UINT_MAX),
-    max_size_amplification_percent(200),
-    compression_size_percent(-1),
-    stop_style(kCompactionStopStyleTotalSize) {
-  }
+  CompactionOptionsUniversal()
+      : size_ratio(1),
+        min_merge_width(2),
+        max_merge_width(UINT_MAX),
+        max_size_amplification_percent(200),
+        compression_size_percent(-1),
+        stop_style(kCompactionStopStyleTotalSize) {}
 };
 
 }  // namespace rocksdb