This looks suspicious - may or may not be the cause. Something is (probably) doing a huge amount of disk reads. AliYunDun seems to be the cloud provider’s tooling. AliYunDunMonitor is (possibly) stuck in disk wait, is running at elevated priority, has clocked up a lot of CPU time.
I see someone somewhere has written a script to kill such processes. The tone is angry. (I cannot say whether or not that script is safe, or a good idea.)
But it’s also possible some Discourse process is doing all the disk reads.
Perhaps try
apt-get install iotop
iotop -b -d 15 -P -o
and watch it until you get your freeze.