Re: terminating walsender process due to replication timeout

Hello,

I took a look in postgresql source code. As far as I understood walsender
can send some data to walreceiver regarding some changes and so-called
keepalive messages.
Exactly these keepalive messages walsender sends periodically once per
wal_sender_timeout seconds (once per 1 second in my case) and expects to
get responses from the walreceiver that everything goes ok.

I switched on trace and repeated my test. I found out that walreceiver
starts processing of the keepalive message in 3 seconds after its sending.
As far as I understand it happened because walreceiver was busy by
accepting all the changes from the master and writing it to its standby
WAL logs.
So the standby postgres log was overloaded overfilled by the entries
regarding writing its own WALs:
18642 2019-05-28 05:08:31 EDT 00000 DEBUG: record known xact 559
latestObservedXid 559
18642 2019-05-28 05:08:31 EDT 00000 CONTEXT: WAL redo at 0/112CE9C0 for
Heap/LOCK: off 53: xid 559: flags 0 LOCK_ONLY EXCL_LOCK
18642 2019-05-28 05:08:31 EDT 00000 DEBUG: record known xact 559
latestObservedXid 559
18642 2019-05-28 05:08:31 EDT 00000 CONTEXT: WAL redo at 0/112CE9F8 for
Heap/UPDATE: off 53 xmax 559 ; new off 129 xmax 0
18642 2019-05-28 05:08:31 EDT 00000 DEBUG: record known xact 559
latestObservedXid 559
18642 2019-05-28 05:08:31 EDT 00000 CONTEXT: WAL redo at 0/112CEA48 for
Heap/LOCK: off 54: xid 559: flags 0 LOCK_ONLY EXCL_LOCK
18642 2019-05-28 05:08:31 EDT 00000 DEBUG: record known xact 559
latestObservedXid 559
18642 2019-05-28 05:08:31 EDT 00000 CONTEXT: WAL redo at 0/112CEA80 for
Heap/UPDATE: off 54 xmax 559 ; new off 130 xmax 0
18642 2019-05-28 05:08:31 EDT 00000 DEBUG: record known xact 559
latestObservedXid 559

So because of writing large volume of data it was not able to handle the
next messages quickly. It seems not to be related to network bandwidth or
CPU saturation.

Thereby, I see some kind of a contradiction with the official description
of wal_sender_timeout parameter:
Terminate replication connections that are inactive longer than the
specified number of milliseconds.
This is useful for the sending server to detect a standby crash or network
outage.

During my test the connection between master and standby was active and
there was no any network outage. So according to the description there was
no need to terminate
replication connection.

So, I have some questions:
Is there any way (e. g. via configuration of other) to make the response
time to keepalive messages independent of the amount of data that the
walreceiver has to process?
If there is no such a way is it possible to update wal_sender_timeout
documentation so it reflects reality?

Best regards,
Andrei

From: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
To: AYahorau(at)ibagroup(dot)eu,
Cc: pgsql-general(at)postgresql(dot)org, rene(dot)romero(dot)b(at)gmail(dot)com
Date: 24/05/2019 09:42
Subject: Re: terminating walsender process due to replication
timeout

Hello.

At Fri, 17 May 2019 11:04:58 +0300, AYahorau(at)ibagroup(dot)eu wrote in
<OFE11EC62A(dot)504EB2B3-ON432583FD(dot)002BE231-432583FD(dot)002C666E(at)iba(dot)by>
> Can frequent database operations cause getting a standby server behind?
Is
> there a way to avoid this situation?
> I checked that walsender works well in my test if I set
> wal_sender_timeout at least to 5 second.

It depends on the transaction (WAL) traffic and the bandwidth of
your network. The performacne difference between master and
standby also affects.

The possibilities I can guess are:

- The bandwidth is narrow to the traffic.

- The standby performs poorer than the master.

- Your network is having a sort of trouble. Virtual network
(local network in a virtual environment) tends to suffer
network troubles caused by CPU saturation or something else.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center