No need to limit to 20 files in UpdateAccumulatedStats() if options.max_open_files=-1

Summary:
There is a hardcoded constraint in our statistics collection that prevents reading properties from more than 20 SST files. This means our statistics will be very inaccurate for databases with > 20 files since additional files are just ignored. The purpose of constraining the number of files used is to bound the I/O performed during statistics collection, since these statistics need to be recomputed every time the database reopened.

However, this constraint doesn't take into account the case where option "max_open_files" is -1. In that case, all the file metadata has already been read, so MaybeInitializeFileMetaData() won't incur any I/O cost. so this diff gets rid of the 20-file constraint in case max_open_files == -1.

Test Plan:
write into unit test db/db_properties_test.cc - "ValidateSampleNumber".
We generate 20 files with 2 rows and 10 files with 1 row.
If max_open_files !=-1, the `rocksdb.estimate-num-keys` should be (10*1 + 10*2)/20 * 30 = 45. Otherwise, it should be the ground truth, 50.
{F1089153}

Reviewers: andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D56253

db/db_properties_test.cc

@@ -253,6 +253,35 @@ TEST_F(DBPropertiesTest, ValidatePropertyInfo) {
   }
 }
 
+TEST_F(DBPropertiesTest, ValidateSampleNumber) {
+  // When "max_open_files" is -1, we read all the files for
+  // "rocksdb.estimate-num-keys" computation, which is the ground truth.
+  // Otherwise, we sample 20 newest files to make an estimation.
+  // Formula: lastest_20_files_active_key_ratio * total_files
+  Options options = CurrentOptions();
+  options.disable_auto_compactions = true;
+  options.level0_stop_writes_trigger = 1000;
+  DestroyAndReopen(options);
+  int key = 0;
+  for (int files = 20; files >= 10; files -= 10) {
+    for (int i = 0; i < files; i++) {
+      int rows = files / 10;
+      for (int j = 0; j < rows; j++) {
+        db_->Put(WriteOptions(), std::to_string(++key), "foo");
+      }
+      db_->Flush(FlushOptions());
+    }
+  }
+  std::string num;
+  Reopen(options);
+  ASSERT_TRUE(dbfull()->GetProperty("rocksdb.estimate-num-keys", &num));
+  ASSERT_EQ("45", num);
+  options.max_open_files = -1;
+  Reopen(options);
+  ASSERT_TRUE(dbfull()->GetProperty("rocksdb.estimate-num-keys", &num));
+  ASSERT_EQ("50", num);
+}
+
 TEST_F(DBPropertiesTest, AggregatedTableProperties) {
   for (int kTableCount = 40; kTableCount <= 100; kTableCount += 30) {
     const int kKeysPerTable = 100;

db/version_set.cc

@@ -1079,6 +1079,12 @@ void Version::UpdateAccumulatedStats(bool update_stats) {
         if (MaybeInitializeFileMetaData(file_meta)) {
           // each FileMeta will be initialized only once.
           storage_info_.UpdateAccumulatedStats(file_meta);
+          // when option "max_open_files" is -1, all the file metadata has
+          // already been read, so MaybeInitializeFileMetaData() won't incur
+          // any I/O cost.
+          if (vset_->db_options_->max_open_files == -1) {
+            continue;
+          }
           if (++init_count >= kMaxInitCount) {
             break;
           }