Use L1 size as estimate for L0 size in LevelCompactionBuilder::GetPathID

Summary:
Fix for [2461](https://github.com/facebook/rocksdb/issues/2461).

Problem: When using multiple db_paths setting with RocksDB, RocksDB incorrectly calculates the size of L1 in LevelCompactionBuilder::GetPathId.

max_bytes_for_level_base is used as L0 size and L1 size is calculated as (L0 size * max_bytes_for_level_multiplier). However, L1 size should be max_bytes_for_level_base.

Solution: Use max_bytes_for_level_base as L1 size. Also, use L1 size as the estimated size of L0.
Closes https://github.com/facebook/rocksdb/pull/2903

Differential Revision: D5885442

Pulled By: maysamyabandeh

fbshipit-source-id: 036da1c9298d173b9b80479cc6661ee4b7a951f6

db/compaction_picker.cc

@@ -1268,6 +1268,8 @@ uint32_t LevelCompactionBuilder::GetPathId(
   uint64_t level_size;
   int cur_level = 0;
 
+  // max_bytes_for_level_base denotes L1 size.
+  // We estimate L0 size to be the same as L1.
   level_size = mutable_cf_options.max_bytes_for_level_base;
 
   // Last path is the fallback
@@ -1278,8 +1280,10 @@ uint32_t LevelCompactionBuilder::GetPathId(
         return p;
       } else {
         current_path_size -= level_size;
-        level_size = static_cast<uint64_t>(
-            level_size * mutable_cf_options.max_bytes_for_level_multiplier);
+        if (cur_level > 0) {
+          level_size = static_cast<uint64_t>(
+              level_size * mutable_cf_options.max_bytes_for_level_multiplier);
+        }
         cur_level++;
         continue;
       }