Issue

Description of problem: We were using "journalctl -f" in the OpenShift MCO operator to watch for login events and we noticed that we don't get any (I suspect because we don't ever reach 50+ logins to a RHCOS box) If you: run "journalctl -f" with a MESSAGE_ID= AND there are no matching log messages already in the journal at the time the command is run Then: once messages with that MESSAGE_ID do exist, you will not receive them until some number/amount/size of them accumulate Version-Release number of selected component (if applicable): systemd 239 (239-45.el8_4.2) on RHCOS systemd 248 (v248.7-1.fc34) on FCOS How reproducible: Every time. Steps to Reproduce: 1. Create a unit that logs filtered logs to a text file: sudo cat << EOF > /etc/systemd/system/early_listen.service [Unit] Description=Hang journalctl before anyone logs in by filtering both unit and message id Before=network-online.target [Service] User=root ExecStart=/bin/sh -c "journalctl -b -f -o cat -u systemd-logind.service MESSAGE_ID=8d45620c1a4348dbb17410da57c60c66 | tee /tmp/journal_unit_messageid.txt" [Install] WantedBy=multi-user.target EOF 2.) Enable that unit systemctl enable early_listen 3.) Clear your journal: sudo journalctl --flush sudo journalctl --vacuum-time=1s 4.) Restart your server (may be able to get by without rebooting, I just always did) sudo systemctl reboot 5.) When the server comes back up, login/ssh into the server ssh core@192.168.122.16 6.) observe that /tmp/journal_unit_messageid.txt is still empty cat /tmp/journal_unit_messageid.txt 7.) if you repeatedly login in a loop, after some number of logins (usually > 50) it will snap out of it and events start coming out immediately Actual results: Messages do not come out until some unseen/unknown threshold is met Expected results: Messages come out as they occur Additional info: It feels like buffering, and at first I thought it was standard output buffering and I was doing something wrong, but after testing I don't think so. I also ran it with "stdbuf" for good measure and it didn't help. If MESSAGE_ID is omitted, journalctl works as expected and messages are returned immediately Any "journalctl -f" process started after a login message is present in the journal does return (and continue to return) events immediately It does not appear to be time based – the messages do not seem to come out after days or hours, the only time they start coming out again is when there seem to be "enough of them"

Release Notes

No. of Comments

Resolution

Unresolved