Re: Logical Replication speed-up initial data - Mailing list pgsql-performance

Hi, 

On Thu, Aug 5, 2021 at 11:28 AM Vijaykumar Jain <vijaykumarjain.github@gmail.com> wrote:

On Thu, 5 Aug 2021 at 10:27, Nikhil Shetty <nikhil.dba04@gmail.com> wrote:

Hi,

Thank you for the suggestion.

We tried by dropping indexes and it worked faster compared to what we saw earlier. We wanted to know if anybody has done any other changes that helps speed-up initial data load without dropping indexes.

You could leverage pg_basbeackup or pg_dump with parallel jobs 

taken from a Standby (preferably replication paused if pg_dump, anyways 

pg_basebackup should be straight-forward) or taken even from 

Primary, for the purpose of initial data load. 

As you are able to drop indexes and make some schema changes, I would 

assume that you could pause your app temporarily. If that's the case
you may look into the simple steps i am posting here that demonstrates 
pg_dump/pg_restore instead. 

If you cannot pause the app, then, you could look into how you  

could use pg_replication_origin_advance 

Step 1 : Pause App 

Step 2 : Create Publication on the Primary CREATE PUBLICATION <some_pub_name> FOR ALL TABLES;

Step 3 : Create Logical Replication Slot on the Primary SELECT * FROM pg_create_logical_replication_slot('<some_slot_name>', 'pgoutput'); Step 4 : Create Subscription but do not enable the Subscription

CREATE SUBSCRIPTION <some_sub_name> CONNECTION

'host=<some_host> dbname=<some_db> user=postgres

password=secret port=5432' PUBLICATION <some_pub_name>

WITH (copy_data = false, create_slot=false, enabled=false,

slot_name=<some_slot_name>);

Step 5 : Initiate pg_dump. We can take a parallel backup for a faster restore.

$ pg_dump -d <some_db> -Fd -j 4 -n <some_schema> -f <some_unique_directory> -- If its several hundreds of GBs or TBs, you may rather utilize one of your Standby that has been paused from replication using -> select pg_wal_replay_pause();

Step 6 : Don't need to wait until pg_dump completes, you may start the App. 
-- Hope the app does not perform changes that impact the pg_dump or
gets blocked due to pg_dump. 

Step 7 : Restore the dump if you used pg_dump. 
pg_restore -d <some_db> -j <some_numer_of_parallel_jobs> <some_directory> Step 8 : Enable subscription.
ALTER SUBSCRIPTION <some_sub_name> ENABLE;

If you have not stopped your app then you must advance the lsn using 
pg_replication_origin_advance 

These are all hand-written steps while drafting this email, so, 

please test it on your end as some typos or adjustments are definitely expected.

PS: i have not tested this in production level loads, it was just some exp i did on my laptop.

one option would be to use pglogical extension (this was shared by Dharmendra in one the previous mails, sharing the same),

and then use pglogical_create_subscriber cli to create the initial copy via pgbasebackup and then carry on from there.

I ran the test case similar to one below in my local env, and it seems to work fine. of course i do not have TB worth of load to test, but it looks promising,

especially since they introduced it to the core.

pglogical/010_pglogical_create_subscriber.pl at REL2_x_STABLE · 2ndQuadrant/pglogical (github.com)

Once you attain some reasonable sync state, you can drop the pglogical extension, and check if things continue fine.

I have done something similar when upgrading from 9.6 to 11 using pglogical and then dropping the extension and it was smooth,

maybe you need to try this out and share if things works fine.

and 

The 1-2-3 for PostgreSQL Logical Replication Using an RDS Snapshot - Percona Database Performance Blog

--

Regards,
Avinash Vallarapu (Avi)

CEO,

MigOps, Inc.