Run db_bench on database generated externally (#5017)

Summary:
Added an option, `-use_existing_keys`, which can be set to run
benchmarks against an arbitrary existing database. Now users can
benchmark against their actual database rather than synthetic data.

Before the run begins, it loads all the keys into memory, then uses that
set of keys rather than synthesizing new ones in `GenerateKeyFromInt`.
This is mainly intended for small-scale DBs where the memory consumption
is not a concern.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5017

Differential Revision: D14270303

Pulled By: riversand963

fbshipit-source-id: 6328df9dffb5e19170270dd00a69f4bbe424e5ed

tools/db_bench_tool.cc

@@ -523,6 +523,14 @@ DEFINE_bool(use_existing_db, false, "If true, do not destroy the existing"
             " database.  If you set this flag and also specify a benchmark that"
             " wants a fresh database, that benchmark will fail.");
 
+DEFINE_bool(use_existing_keys, false,
+            "If true, uses existing keys in the DB, "
+            "rather than generating new ones. This involves some startup "
+            "latency to load all keys into memory. It is supported for the "
+            "same read/overwrite benchmarks as `-use_existing_db=true`, which "
+            "must also be set for this flag to be enabled. When this flag is "
+            "set, the value for `-num` will be ignored.");
+
 DEFINE_bool(show_table_properties, false,
             "If true, then per-level table"
             " properties will be printed on every stats-interval when"
@@ -700,6 +708,7 @@ DEFINE_string(
     "RocksDB options related command-line arguments, all other arguments "
     "that are related to RocksDB options will be ignored:\n"
     "\t--use_existing_db\n"
+    "\t--use_existing_keys\n"
     "\t--statistics\n"
     "\t--row_cache_size\n"
     "\t--row_cache_numshardbits\n"
@@ -2051,6 +2060,7 @@ class Benchmark {
   int64_t merge_keys_;
   bool report_file_operations_;
   bool use_blob_db_;
+  std::vector<std::string> keys_;
 
   class ErrorHandlerListener : public EventListener {
    public:
@@ -2470,6 +2480,13 @@ class Benchmark {
   //   |        key 00000         |
   //   ----------------------------
   void GenerateKeyFromInt(uint64_t v, int64_t num_keys, Slice* key) {
+    if (!keys_.empty()) {
+      assert(FLAGS_use_existing_keys);
+      assert(keys_.size() == static_cast<size_t>(num_keys));
+      assert(v < static_cast<uint64_t>(num_keys));
+      *key = keys_[v];
+      return;
+    }
     char* start = const_cast<char*>(key->data());
     char* pos = start;
     if (keys_per_prefix_ > 0) {
@@ -3654,6 +3671,19 @@ void VerifyDBFromDB(std::string& truth_db_name) {
       options.compaction_filter = new KeepFilter();
       fprintf(stdout, "A noop compaction filter is used\n");
     }
+
+    if (FLAGS_use_existing_keys) {
+      // Only work on single database
+      assert(db_.db != nullptr);
+      ReadOptions read_opts;
+      read_opts.total_order_seek = true;
+      Iterator* iter = db_.db->NewIterator(read_opts);
+      for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {
+        keys_.emplace_back(iter->key().ToString());
+      }
+      delete iter;
+      FLAGS_num = keys_.size();
+    }
   }
 
   void Open(Options* opts) {
@@ -6113,6 +6143,13 @@ int db_bench_tool(int argc, char** argv) {
     }
   }
 #endif  // ROCKSDB_LITE
+  if (FLAGS_use_existing_keys && !FLAGS_use_existing_db) {
+    fprintf(stderr,
+            "`-use_existing_db` must be true for `-use_existing_keys` to be "
+            "settable\n");
+    exit(1);
+  }
+
   if (!FLAGS_hdfs.empty()) {
     FLAGS_env  = new rocksdb::HdfsEnv(FLAGS_hdfs);
   }