Subject: Replication lag in Postgres

From: Mukesh Tanuku <[email-masked]>
Date: Fri, 12 Jul 2024 20:41:03 +0530

Hello everyone.
Firstly thanks to the community members who are addressing all the queries
that are posted. Those give us more insights about the issues/doubts in the
postgres.

I have a question with postgres HA setup.
We are setting up a 2 node postgres cluster with async streaming
replication, we want to define a RPO (Recovery point objective) in case of
primary failure.

How can we best define the RPO in this setup? since it's an async streaming
replication setup there might be a chance of data loss which is
proportional to the replication delay.

Is there any way we can configure the delay duration, like for example to
make sure every 10 mins the standby sync has to happen with primary?

Thank you
Regards
Mukesh T

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From: Laurenz Albe <[email-masked]>
Date: Fri, 12 Jul 2024 17:43:24 +0200

On Fri, 2024-07-12 at 20:41 +0530, Mukesh Tanuku wrote:
> We are setting up a 2 node postgres cluster with async streaming replication, we want to
> define a RPO (Recovery point objective) in case of primary failure. 
> 
> How can we best define the RPO in this setup? since it's an async streaming replication
> setup there might be a chance of data loss which is proportional to the replication delay. 
> 
> Is there any way we can configure the delay duration, like for example to make sure every
> 10 mins the standby sync has to happen with primary? 

When there is a delay, it is usually because replay at the standby is delayed.
The WAL information is still replicated.  You won't lose that information on
failover; it will just make the failover take longer.

Unless you have a network problem, you should never lose more than a fraction
of a second.

Yours,
Laurenz Albe

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From: Mukesh Tanuku <[email-masked]>
Date: Sat, 13 Jul 2024 11:32:53 +0530

Thank you for the information Laurenz Albe

On Fri, Jul 12, 2024 at 9:13 PM Laurenz Albe <[email-masked]>
wrote:

> replication, we want to

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From: Muhammad Imtiaz <[email-masked]>
Date: Sat, 13 Jul 2024 14:34:21 +0500

Hi,

I recommend the following configurations/options in this case:

• wal_sender_timeout: This setting determines how long the primary server
waits for the standby server to acknowledge receipt of WAL data. Adjusting
this can help ensure timely data transfer.

• wal_keep_size: Ensures that enough WAL files are retained for the standby
to catch up if it falls behind.

• checkpoint_timeout: Adjust the checkpoint frequency to ensure WAL files
are regularly flushed and sent to the standby server regularly.

• pg_receivewal: Use this tool to continuously archive WAL files to a safe
location.It will helpful if there is a delay in streaming replication, you
have a backup of WAL files.

Regards,
Muhammad Imtiaz

On Fri, 12 Jul 2024, 20:11 Mukesh Tanuku, <[email-masked]> wrote:

> How can we best define the RPO in this setup? since it's an async
> streaming replication setup there might be a chance of data loss which is