- If a client renews before 'dhcp-cache-threshold' percent of its lease

has elapsed (default 25%), the server will reuse the allocated lease
  (provide a lease within the currently allocated lease-time) rather
  than extend or renew the lease.  This absolves the server of needing
  to perform an fsync() operation on the lease database before reply,
  which improves performance. [ISC-Bugs #22228]

server/stables.c

@@ -266,6 +266,7 @@ static struct option server_options[] = {
 	{ "ldap-tls-randfile", "t",		&server_universe,  77, 1 },
 #endif /* LDAP_USE_SSL */
 #endif /* LDAP_CONFIGURATION */
+	{ "dhcp-cache-threshold", "B",		&server_universe,  78, 1 },
 	{ NULL, NULL, NULL, 0, 0 }
 };