All environment info on my os.debian8@debian:~$ uname -aLinux debian 3.16.0-4-amd64 #1 SMP Debian 3.16.39-1+deb8u2 (2017-03-07) x86_64 GNU/Linuxdebian8@debian:~$ systemd --versionsystemd 215+PAM +AUDIT +SELINUX +IMA +SYSVINIT +LIBCRYPTSETUP +GCRYPT +ACL +XZ -SECCOMP -APPARMORMy journald.conf configuration.sudo cat  /etc/systemd/journald.conf | grep -v #[Journal]Storage=persistentCompress=yesSeal=yesSyncIntervalSec=5mSystemMaxUse=100MSystemMaxFileSize=100MHow many journals info on my pc.sudo journalctl --disk-usageJournals take up 8.0M on disk.Now to retain only 4M with such command as below.sudo journalctl --vacuum-size=1Mjournalctl: unrecognized option '--vacuum-size=1M'
journalctl: unrecognized option '--vacuum-size= '
systemd;journald;logs;delete;journalctl