Make --benchmarks=flush flush the default column family (#9887)

Summary:
db_bench --benchmarks=flush wasn't flushing the default column family.

This is for https://github.com/facebook/rocksdb/issues/9880

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9887

Test Plan:
Confirm that flush works (*.log is empty) when "flush" added to benchmark list
Confirm that *.log is not empty otherwise.

Repeat for all combinations for: uses column families, uses multiple databases

./db_bench --benchmarks=overwrite --num=10000
ls -lrt /tmp/rocksdbtest-2260/dbbench/*.log
-rw-r--r-- 1 me users 1380286 Apr 21 10:47 /tmp/rocksdbtest-2260/dbbench/000004.log

./db_bench --benchmarks=overwrite,flush --num=10000
ls -lrt /tmp/rocksdbtest-2260/dbbench/*.log
 -rw-r--r-- 1 me users 0 Apr 21 10:48 /tmp/rocksdbtest-2260/dbbench/000008.log

./db_bench --benchmarks=overwrite --num=10000 --num_column_families=4
ls -lrt /tmp/rocksdbtest-2260/dbbench/*.log
  -rw-r--r-- 1 me users 1387823 Apr 21 10:49 /tmp/rocksdbtest-2260/dbbench/000004.log

./db_bench --benchmarks=overwrite,flush --num=10000 --num_column_families=4
ls -lrt /tmp/rocksdbtest-2260/dbbench/*.log
-rw-r--r-- 1 me users 0 Apr 21 10:51 /tmp/rocksdbtest-2260/dbbench/000014.log

./db_bench --benchmarks=overwrite --num=10000 --num_multi_db=2
ls -lrt /tmp/rocksdbtest-2260/dbbench/[01]/*.log
 -rw-r--r-- 1 me users 1380838 Apr 21 10:55 /tmp/rocksdbtest-2260/dbbench/0/000004.log
 -rw-r--r-- 1 me users 1379734 Apr 21 10:55 /tmp/rocksdbtest-2260/dbbench/1/000004.log

./db_bench --benchmarks=overwrite,flush --num=10000 --num_multi_db=2
ls -lrt /tmp/rocksdbtest-2260/dbbench/[01]/*.log
-rw-r--r-- 1 me users 0 Apr 21 10:57 /tmp/rocksdbtest-2260/dbbench/0/000013.log
-rw-r--r-- 1 me users 0 Apr 21 10:57 /tmp/rocksdbtest-2260/dbbench/1/000013.log

./db_bench --benchmarks=overwrite --num=10000 --num_column_families=4 --num_multi_db=2
ls -lrt /tmp/rocksdbtest-2260/dbbench/[01]/*.log
-rw-r--r-- 1 me users 1395108 Apr 21 10:52 /tmp/rocksdbtest-2260/dbbench/1/000004.log
-rw-r--r-- 1 me users 1380411 Apr 21 10:52 /tmp/rocksdbtest-2260/dbbench/0/000004.log

./db_bench --benchmarks=overwrite,flush --num=10000 --num_column_families=4 --num_multi_db=2
ls -lrt /tmp/rocksdbtest-2260/dbbench/[01]/*.log
-rw-r--r-- 1 me users 0 Apr 21 10:54 /tmp/rocksdbtest-2260/dbbench/0/000022.log
-rw-r--r-- 1 me users 0 Apr 21 10:54 /tmp/rocksdbtest-2260/dbbench/1/000022.log

Reviewed By: ajkr

Differential Revision: D36026777

Pulled By: mdcallag

fbshipit-source-id: d42d3d7efceea7b9a25bbbc0f04461d2b7301122

tools/db_bench_tool.cc

@@ -8021,14 +8021,27 @@ class Benchmark {
     flush_opt.wait = true;
 
     if (db_.db != nullptr) {
-      Status s = db_.db->Flush(flush_opt, db_.cfh);
+      Status s;
+      if (FLAGS_num_column_families > 1) {
+        s = db_.db->Flush(flush_opt, db_.cfh);
+      } else {
+        s = db_.db->Flush(flush_opt, db_.db->DefaultColumnFamily());
+      }
+
       if (!s.ok()) {
         fprintf(stderr, "Flush failed: %s\n", s.ToString().c_str());
         exit(1);
       }
     } else {
       for (const auto& db_with_cfh : multi_dbs_) {
-        Status s = db_with_cfh.db->Flush(flush_opt, db_with_cfh.cfh);
+        Status s;
+        if (FLAGS_num_column_families > 1) {
+          s = db_with_cfh.db->Flush(flush_opt, db_with_cfh.cfh);
+        } else {
+          s = db_with_cfh.db->Flush(flush_opt,
+                                    db_with_cfh.db->DefaultColumnFamily());
+        }
+
         if (!s.ok()) {
           fprintf(stderr, "Flush failed: %s\n", s.ToString().c_str());
           exit(1);