FLUSHALL will only perform a blocking SAVE if RDB persistence is configured.

src/db.c

@@ -180,7 +180,7 @@ void flushallCommand(redisClient *c) {
         kill(server.bgsavechildpid,SIGKILL);
         rdbRemoveTempFile(server.bgsavechildpid);
     }
-    rdbSave(server.dbfilename);
+    if (server.saveparamslen > 0) rdbSave(server.dbfilename);
     server.dirty++;
 }