Add WARN/INFO for mempurge output status. (facebook#8514)

Summary:
The MemPurge output status can either be an Abort if the mempurge is aborted due to the new_mem memtable reaching more than the target capacity (currently 60%), or for other reasons. As a result, in the log, we want to differentiate between an abort status, which in this PR only leads to a ROCKS_LOG_INFO, and any other status, which in this PR leads to a ROCKS_LOG_WARN.

Pull Request resolved: facebook#8514

Reviewed By: pdillinger

Differential Revision: D29662446

Pulled By: bjlemaire

fbshipit-source-id: c9bec8e238ebc7ecb14fbbddf580e6887e281c16

db/flush_job.cc

@@ -233,9 +233,17 @@ Status FlushJob::Run(LogsWithPrepTracker* prep_tracker,
       (!mems_.empty())) {
     mempurge_s = MemPurge();
     if (!mempurge_s.ok()) {
-      ROCKS_LOG_INFO(db_options_.info_log,
-                     "Mempurge process unsuccessful: %s\n",
-                     mempurge_s.ToString().c_str());
+      // Mempurge is typically aborted when the new_mem output memtable
+      // is filled at more than XX % capacity (currently: 60%).
+      if (mempurge_s.IsAborted()) {
+        ROCKS_LOG_INFO(db_options_.info_log, "Mempurge process aborted: %s\n",
+                       mempurge_s.ToString().c_str());
+      } else {
+        // However the mempurge process can also fail for
+        // other reasons (eg: new_mem->Add() fails).
+        ROCKS_LOG_WARN(db_options_.info_log, "Mempurge process failed: %s\n",
+                       mempurge_s.ToString().c_str());
+      }
     }
   }
   Status s;