小块写缓存优化

test/test_single_file_perf.c

@@ -3,6 +3,7 @@
 static int test_single_file_perf(const char *path)
 {
     size_t io_size = 128 * 1024;
+    // size_t io_size = 4096;
     size_t max_size = 2ULL * 1024 * 1024 * 1024;
     size_t max_count = max_size / io_size;
     int test_sec = 10;