10min cpu iowait

[Chris]

Ubuntu 18.04.5LTS, kernel 5.4.0-42-generic
I've been running 'netdata' for a little over a week, lately I've been
seeing this in the alarm logs:

"3:32:49 AM",system.cpu,"10min cpu iowait",16.9%,CLEAR
"3:19:49 AM",system.cpu,"10min cpu iowait",40.9%,WARNING
"8/7/2020 3:28:48 AM",system.cpu,"10min cpu iowait",16.8%,CLEAR
"8/7/2020 3:19:48 AM",system.cpu,"10min cpu iowait",41.1%,WARNING
"8/4/2020 3:31:46 AM",system.cpu,"10min cpu iowait",16.2%,CLEAR
"8/4/2020 3:19:46 AM",system.cpu,"10min cpu iowait",40.5%,WARNING
"8/3/2020 3:26:45 AM",system.cpu,"10min cpu iowait",16.9%,CLEAR
"8/3/2020 3:19:45 AM",system.cpu,"10min cpu iowait",41.8%,WARNING

I've looked up iowait and understand what it is however I'm not sure
how to find the cause or where to look. Nothing really suspicious in
the syslog. I do have an rsync backup to an external usb drive that
runs starting at 3:10am and runs for about 15 minutes or so.  I'm
puzzled because it's not happening every day. I've pasted syslog output
here - https://pastebin.com/6aVe2PUu during the time just prior to the
warning from netdata.

If any more information is required let me know.

-- 
Chris
31.11972; -97.90167 (Elev. 1092 ft)
10:35:07 up 17 days, 1:57, 1 user, load average: 2.05, 1.53, 1.01
Description:	Ubuntu 18.04.5 LTS, kernel 5.4.0-42-generic