Flush stale column families less aggressively

Summary:
We've seen some production issues where column family is detected as stale, although there is only one column family in the system. This is a quick fix that:
1) doesn't flush stale column families if there's only one of them
2) Use 4 as a coefficient instead of 2 for determening when a column family is stale. This will make flushing less aggressive, while still keep a nice dynamic flushing of very stale CFs.

Test Plan: make check

Reviewers: dhruba, haobo, ljin, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18861

db/column_family.cc

@@ -506,6 +506,10 @@ void ColumnFamilySet::UpdateMaxColumnFamily(uint32_t new_max_column_family) {
   max_column_family_ = std::max(new_max_column_family, max_column_family_);
 }
 
+size_t ColumnFamilySet::NumberOfColumnFamilies() const {
+  return column_families_.size();
+}
+
 // under a DB mutex
 ColumnFamilyData* ColumnFamilySet::CreateColumnFamily(
     const std::string& name, uint32_t id, Version* dummy_versions,

db/column_family.h

@@ -302,8 +302,8 @@ class ColumnFamilyData {
 // family might get dropped when the DB mutex is released
 // * GetDefault() -- thread safe
 // * GetColumnFamily() -- either inside of DB mutex or call Lock() <-> Unlock()
-// * GetNextColumnFamilyID(), GetMaxColumnFamily(), UpdateMaxColumnFamily() --
-// inside of DB mutex
+// * GetNextColumnFamilyID(), GetMaxColumnFamily(), UpdateMaxColumnFamily(),
+// NumberOfColumnFamilies -- inside of DB mutex
 class ColumnFamilySet {
  public:
   // ColumnFamilySet supports iteration
@@ -342,6 +342,7 @@ class ColumnFamilySet {
   uint32_t GetNextColumnFamilyID();
   uint32_t GetMaxColumnFamily();
   void UpdateMaxColumnFamily(uint32_t new_max_column_family);
+  size_t NumberOfColumnFamilies() const;
 
   ColumnFamilyData* CreateColumnFamily(const std::string& name, uint32_t id,
                                        Version* dummy_version,

db/db_impl.cc

@@ -3684,15 +3684,17 @@ Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {
 
   uint64_t flush_column_family_if_log_file = 0;
   uint64_t max_total_wal_size = (options_.max_total_wal_size == 0)
-                                    ? 2 * max_total_in_memory_state_
+                                    ? 4 * max_total_in_memory_state_
                                     : options_.max_total_wal_size;
-  if (alive_log_files_.begin()->getting_flushed == false &&
+  if (versions_->GetColumnFamilySet()->NumberOfColumnFamilies() > 1 &&
+      alive_log_files_.begin()->getting_flushed == false &&
       total_log_size_ > max_total_wal_size) {
     flush_column_family_if_log_file = alive_log_files_.begin()->number;
     alive_log_files_.begin()->getting_flushed = true;
     Log(options_.info_log,
-        "Flushing all column families with data in WAL number %" PRIu64,
-        flush_column_family_if_log_file);
+        "Flushing all column families with data in WAL number %" PRIu64
+        ". Total log size is %" PRIu64 " while max_total_wal_size is %" PRIu64,
+        flush_column_family_if_log_file, total_log_size_, max_total_wal_size);
   }
 
   Status status;