Suddenly all queries moved to seq scan

Hi,

We are using PostgresQL 10 in our production database. We have around 890
req /s request on peak time.

We have 1 primary and 4 slave databases as well in the same postgres
cluster.

2 days back we applied some patches in the primary server and restarted. We
didn't do anything on the secondary server.

Next day, After 18 hours all our queries from secondary servers started
taking too much time. queries were working in 2 sec started taking 80
seconds. Almost all queries behaved the same way.

After half an hour of outage we restarted all db servers and system back to
normal.

Still we are not able to understand the root case. We couldn't find any
error log or fatal errors. During the incident, in one of the read server
disks was full. We couldn't see any replication lag or query
cancellation due to replication.

please help

Regards
Sreejith

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