CACHEFILES_DIO_BLOCK_SIZE
ni_size = round_up(ni_size, CACHEFILES_DIO_BLOCK_SIZE);
dio_size = round_up(new_size, CACHEFILES_DIO_BLOCK_SIZE);
off = start & (CACHEFILES_DIO_BLOCK_SIZE - 1);
pre = CACHEFILES_DIO_BLOCK_SIZE - off;
size_t part = len % CACHEFILES_DIO_BLOCK_SIZE;
size_t need = CACHEFILES_DIO_BLOCK_SIZE - part;
post = len & (CACHEFILES_DIO_BLOCK_SIZE - 1);
ni_size = round_up(ni_size, CACHEFILES_DIO_BLOCK_SIZE);