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On my Ubuntu 21.10 Ubuntu desktop, which is sometimes on at midnight (if I happen to be awake and using it), a nightly event takes place where there is a lot of disk activity -- the red light for the HDD is blinking repeatedly, even though I'm not doing anything. I don't know what it is, but it does cause the system to slow down.

I know it isn't a user-level or a root-level cronjob. I've checked that. So, I guess it's something that has been turned on by default. I suppose I also have a slight concern that it's something done by an outside party, but I guess that's unlikely since midnight every night is a bit too predictable.

I have set up a backup program, but that program doesn't run at midnight. It's possible it's just rotating log files or something. Whatever it is, I just want to know what it is (and maybe move it to another time).

Anyway, what can I do to figure out what's causing this? I have looked at the output of top, but nothing looks out of place. Besides the user and root's crontab, is there anything else I should check?

Any suggestions would be appreciated!

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    Did you try running a resource monitor (like htop) during this time, and see which process consumes most CPU? Mar 6, 2022 at 17:59
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    You're experiencing some "cron" jobs, or scheduled timer jobs that do some housekeeping. Check with systemctl list-timers (maybe also crontab -l) the list of timers. They will also show you when they start again
    – kanehekili
    Mar 6, 2022 at 18:27
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    What do your logs for that time say?
    – user535733
    Mar 6, 2022 at 18:35
  • @kanehekili Thank you! I didn't know about systemctl list-timers. (It wasn't anything I set in crontab, though.) Combined with the answer below, I was able to find the culprit to be mlocate. Thank you!
    – Ray
    Mar 8, 2022 at 9:42

1 Answer 1

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Look at the logs:

# Use the power of the date command to produce journalctl-friendly date format
alias tsjou='date '\''+%y-%m-%d %H:%M:%S'\'''

Examples:

walt@bat:~(0)$ tsjou
22-03-06 14:02:25
walt@bat:~(0)$ tsjou --date="23:45 yesterday"
22-03-05 23:45:00

To see the system logs around midnight:

sudo journalctl --since="$(tsjou --date="23:45 yesterday")" \
                --until="$(tsjou --date="00:15")"

Read man date journalctl. I have more journalctl hints at https://askubuntu.com/users/25618/waltinator?tab=profile You'll have to click the "Read more" button - I'm verbose.

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  • Thank you for this! I know about journalctl, but not well enough to make it useful for me. Combining your suggestion with systemctl list-timers in the comments above, I found out that rotating logs, man-db regeneration, and mlocate were running simultaneously. The first two finish in 30 seconds. mlocate runs for about 20 minutes but says it consumed 1 min of CPU time. I guess it has a lot of disk I/O...I will have to do something about it. I did find this just now. Thank you for reply and the hints!
    – Ray
    Mar 8, 2022 at 9:39

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