[lmdb facebookincubator#10] Set mapsize to 1TB by default for r/w DBs

Author: simonmar

glean/lmdb/container-impl.cpp

@@ -74,7 +74,19 @@ ContainerImpl::ContainerImpl(
   MDB_env *db_ptr;
   check( mdb_env_create(&db_ptr) );
   check( mdb_env_set_maxdbs(db_ptr, Family::count()) );
-  check( mdb_env_set_mapsize(db_ptr, 10*1024*1024*1024UL) );
+
+  // LMDB needs us to pre-set the memory map size, which determines
+  // the maximum size of the DB. Setting it to zero uses the mapsize
+  // when the DB was created, while setting it to a value smaller than
+  // the actual size of the DB causes LMDB to use the DB size - this
+  // is what we want for a read-only DB. For a read/write DB we have
+  // to pick a huge value, because the mapsize can only be increased
+  // if there are no transactions in progress, and we don't have a good
+  // way to do that aside from closing and re-opening the DB.
+  check( mdb_env_set_mapsize(
+             db_ptr,
+             m == Mode::ReadOnly ? 1 : 1024*1024*1024*1024UL) );
+
   check( mdb_env_open(db_ptr, path.c_str(), flags, 0666) );
   SCOPE_FAIL { mdb_env_close(db_ptr); };