Add rocksdb.num-live-versions: number of live versions

Summary: Add a DB property about live versions. It can be helpful to figure out whether there are files not live but not yet deleted, in some use cases.

Test Plan: make all check

Reviewers: rven, yhchiang, igor

Reviewed By: igor

Subscribers: yoshinorim, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D33327

db/column_family.cc

@@ -439,6 +439,10 @@ void ColumnFamilyData::SetCurrent(Version* current_version) {
   current_ = current_version;
 }
 
+uint64_t ColumnFamilyData::GetNumLiveVersions() const {
+  return VersionSet::GetNumLiveVersions(dummy_versions_);
+}
+
 MemTable* ColumnFamilyData::ConstructNewMemtable(
     const MutableCFOptions& mutable_cf_options) {
   assert(current_ != nullptr);

db/column_family.h

@@ -207,6 +207,8 @@ class ColumnFamilyData {
   Version* current() { return current_; }
   Version* dummy_versions() { return dummy_versions_; }
   void SetCurrent(Version* current);
+  uint64_t GetNumLiveVersions() const;  // REQUIRE: DB mutex held
+
   MemTable* ConstructNewMemtable(const MutableCFOptions& mutable_cf_options);
   void SetMemtable(MemTable* new_mem) { mem_ = new_mem; }
   void CreateNewMemtable(const MutableCFOptions& mutable_cf_options);

db/db_test.cc

@@ -3230,6 +3230,43 @@ TEST(DBTest, GetProperty) {
   ASSERT_TRUE(
       dbfull()->GetIntProperty("rocksdb.estimate-table-readers-mem", &int_num));
   ASSERT_GT(int_num, 0U);
+
+  // Test rocksdb.num-live-versions
+  {
+    options.level0_file_num_compaction_trigger = 20;
+    Reopen(options);
+    ASSERT_TRUE(
+        dbfull()->GetIntProperty("rocksdb.num-live-versions", &int_num));
+    ASSERT_EQ(int_num, 1U);
+
+    // Use an iterator to hold current version
+    std::unique_ptr<Iterator> iter1(dbfull()->NewIterator(ReadOptions()));
+
+    ASSERT_OK(dbfull()->Put(writeOpt, "k6", big_value));
+    Flush();
+    ASSERT_TRUE(
+        dbfull()->GetIntProperty("rocksdb.num-live-versions", &int_num));
+    ASSERT_EQ(int_num, 2U);
+
+    // Use an iterator to hold current version
+    std::unique_ptr<Iterator> iter2(dbfull()->NewIterator(ReadOptions()));
+
+    ASSERT_OK(dbfull()->Put(writeOpt, "k7", big_value));
+    Flush();
+    ASSERT_TRUE(
+        dbfull()->GetIntProperty("rocksdb.num-live-versions", &int_num));
+    ASSERT_EQ(int_num, 3U);
+
+    iter2.reset();
+    ASSERT_TRUE(
+        dbfull()->GetIntProperty("rocksdb.num-live-versions", &int_num));
+    ASSERT_EQ(int_num, 2U);
+
+    iter1.reset();
+    ASSERT_TRUE(
+        dbfull()->GetIntProperty("rocksdb.num-live-versions", &int_num));
+    ASSERT_EQ(int_num, 1U);
+  }
 }
 
 TEST(DBTest, FLUSH) {

db/internal_stats.cc

@@ -140,6 +140,8 @@ DBPropertyType GetPropertyType(const Slice& property, bool* is_int_property,
     return kNumSnapshots;
   } else if (in == "oldest-snapshot-time") {
     return kOldestSnapshotTime;
+  } else if (in == "num-live-versions") {
+    return kNumLiveVersions;
   }
   return kUnknown;
 }
@@ -224,6 +226,7 @@ bool InternalStats::GetStringProperty(DBPropertyType property_type,
 
 bool InternalStats::GetIntProperty(DBPropertyType property_type,
                                    uint64_t* value, DBImpl* db) const {
+  db->mutex_.AssertHeld();
   const auto* vstorage = cfd_->current()->storage_info();
 
   switch (property_type) {
@@ -273,6 +276,9 @@ bool InternalStats::GetIntProperty(DBPropertyType property_type,
     case kOldestSnapshotTime:
       *value = static_cast<uint64_t>(db->snapshots().GetOldestSnapshotTime());
       return true;
+    case kNumLiveVersions:
+      *value = cfd_->GetNumLiveVersions();
+      return true;
 #ifndef ROCKSDB_LITE
     case kIsFileDeletionEnabled:
       *value = db->IsFileDeletionsEnabled();

db/internal_stats.h

@@ -50,6 +50,7 @@ enum DBPropertyType : uint32_t {
                                   // 0 means file deletions enabled
   kNumSnapshots,                  // Number of snapshots in the system
   kOldestSnapshotTime,            // Unix timestamp of the first snapshot
+  kNumLiveVersions,
 };
 
 extern DBPropertyType GetPropertyType(const Slice& property,

db/version_set.cc

@@ -2807,4 +2807,12 @@ ColumnFamilyData* VersionSet::CreateColumnFamily(
   return new_cfd;
 }
 
+uint64_t VersionSet::GetNumLiveVersions(Version* dummy_versions) {
+  uint64_t count = 0;
+  for (Version* v = dummy_versions->next_; v != dummy_versions; v = v->next_) {
+    count++;
+  }
+  return count;
+}
+
 }  // namespace rocksdb

db/version_set.h

@@ -596,6 +596,8 @@ class VersionSet {
   ColumnFamilySet* GetColumnFamilySet() { return column_family_set_.get(); }
   const EnvOptions& env_options() { return env_options_; }
 
+  static uint64_t GetNumLiveVersions(Version* dummy_versions);
+
  private:
   struct ManifestWriter;
 

include/rocksdb/db.h

@@ -325,6 +325,10 @@ class DB {
   //  "rocksdb.is-file-deletions-enabled"
   //  "rocksdb.num-snapshots"
   //  "rocksdb.oldest-snapshot-time"
+  //  "rocksdb.num-live-versions" - `version` is an internal data structure.
+  //      See version_set.h for details. More live versions often mean more SST
+  //      files are held from being deleted, by iterators or unfinished
+  //      compactions.
   virtual bool GetProperty(ColumnFamilyHandle* column_family,
                            const Slice& property, std::string* value) = 0;
   virtual bool GetProperty(const Slice& property, std::string* value) {
@@ -347,6 +351,7 @@ class DB {
   //  "rocksdb.is-file-deletions-enabled"
   //  "rocksdb.num-snapshots"
   //  "rocksdb.oldest-snapshot-time"
+  //  "rocksdb.num-live-versions"
   virtual bool GetIntProperty(ColumnFamilyHandle* column_family,
                               const Slice& property, uint64_t* value) = 0;
   virtual bool GetIntProperty(const Slice& property, uint64_t* value) {