Function sd_journal_set_data_threshold 

pub unsafe extern "C" fn sd_journal_set_data_threshold(
    journal: *mut sd_journal,
    size: size_t,
) -> c_int

int sd_journal_set_data_threshold(sd_journal *j, size_t sz);

https://www.freedesktop.org/software/systemd/man/sd_journal_get_data.html#