sd_journal_get_usage — Journal disk usage
sd_journal_get_usage() determines the
total disk space currently used by journal files (in bytes). If
SD_JOURNAL_LOCAL_ONLY was passed when opening
the journal, this value will only reflect the size of journal
files of the local host, otherwise of all hosts.
sd_journal_get_usage() returns 0 on
success or a negative errno-style error code.
sd_journal_get_usage() interface is
available as a shared library, which can be compiled and linked to