How to clear system log?

To get stat about system storage

sudo du -h --max-depth=1 / 2>/dev/null | sort -h

Result:

0	/dev
0	/proc
0	/sys
4.0K	/home
4.0K	/mnt
4.0K	/srv
8.0K	/build
12K	/media
16K	/lost+found
60K	/tmp
192K	/ephe
2.9M	/run
5.8M	/etc
164M	/opt
219M	/boot
841M	/snap
3.2G	/usr
15G	/root
27G	/var
51G	/

Check the var directory with sudo du -h --max-depth=1 /var 2>/dev/null | sort -h. Result:

4.0K	/var/crash
4.0K	/var/local
4.0K	/var/mail
4.0K	/var/opt
36K	/var/tmp
48K	/var/snap
1.7M	/var/backups
2.6M	/var/spool
114M	/var/cache
7.7G	/var/log
22G	/var/lib
30G	/var

To clear system log, under /var/log. To leave the latest 500MB:

journalctl --vacuum-size=500M

Result:

Vacuuming done, freed 0B of archived journals from /run/log/journal.
Deleted archived journal /var/log/journal/6493b9faf8566e8b87835d996022a956/system@685e3a233a8d47b19c3ccd164e23af96-00000000013c2c59-0005fec131c087c3.journal (16.0M).
Deleted archived journal /var/log/journal/6493b9faf8566e8b87835d996022a956/system@685e3a233a8d47b19c3ccd164e23af96-00000000013c6746-0005fec141f2d31d.journal (128.0M).
Deleted archived journal /var/log/journal/6493b9faf8566e8b87835d996022a956/system@685e3a233a8d47b19c3ccd164e23af96-00000000013ed1fd-0005fec292ba2aec.journal (128.0M).
Deleted archived journal /var/log/journal/6493b9faf8566e8b87835d996022a956/system@685e3a233a8d47b19c3ccd164e23af96-000000000141428d-0005fec344f94bf2.journal (128.0M).
Deleted archived journal /var/log/journal/6493b9faf8566e8b87835d996022a956/system@685e3a233a8d47b19c3ccd164e23af96-000000000143b3b6-0005fec40d477878.journal (128.0M).
Deleted archived journal /var/log/journal/6493b9faf8566e8b87835d996022a956/system@685e3a233a8d47b19c3ccd164e23af96-00000000014624dc-0005fec4e23fe94c.journal (128.0M).
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Vacuuming done, freed 2.8G of archived journals from /var/log/journal/6493b9faf8566e8b87835d996022a956.
Vacuuming done, freed 0B of archived journals from /var/log/journal.

To keep last 2 days log only: journalctl --vacuum-time=2d.