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.kyotaro@xxxxxxxxxxxxx>
To:
AYahorau@xxxxxxxxxxx,
Cc:
pgsql-general@xxxxxxxxxxxxxx,
rene.romero.b@xxxxxxxxx
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@xxxxxxxxxxx wrote in <OFE11EC62A.504EB2B3-ON432583FD.002BE231-432583FD.002C666E@xxxxxx>
> 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
