pg_upgrade - Easiest way to upgrade PostgreSQL cluster

In this blog, I am going to provide details steps to do upgrades a PostgreSQL cluster to a different major version (for instance PG v11 to PG v12) .
There are few  conditions which  need to be meet-
A)Both clusters should not be running (so little down time is required but this is the most efficient way) to upgrade PostgreSQL version from X major version to Y major version.
B)A new database cluster (using the new version of initdb) for  PostgreSQL version which we need to migrate.

These below are the available options of pg_upgrade utility -

Lets assume - PG v11 is the current server which contain all data/objects.

and after PG v12 release  which is the latest and the greatest contains new features including critical bug fixes -User wants to migrate.

Note- It is always recommended to do  testing on testing box  before touching the live server.

Prerequisites -
First need to make sure - PG v12 is installed on the same machine where PG v11 is installed/Running.
Shutdown the PG v11 server.

Steps to perform pg_upgrade -

Perform initdb ( initdb utility  can be found inside  bin/ directory of PG v12)

When we run pg_upgrade, we should  provide the following information:

 ->data directory for the old cluster  (-d old_data_directory_path)

 ->data directory for the new cluster  (-D new_data_directory_path)

 ->bin directory for the old version (-b old_bin_directory_path)

 ->bin directory for the new version (-B new_bin_directory_path)

For example:

  pg_upgrade -d <old_data_directory_path> -D <new_data_directory_path> -b  <old_bin_directory_path> -B <new_bin_directory_path>

Things to remember-

a)If somehow pg_upgrade get failed and we want to try it again then it is better to perform initdb again (PG v12) . Don't use the existing PG v12 cluster otherwise if we try to run pg_upgrade again without performing  fresh PG v12 - we can get some weird error like -

"

Setting frozenxid and minmxid counters in new cluster  connection to database failed: 

FATAL:  could not open relation with OID 2610

Failure, exiting

"

b)Always recommend to use dry run (option -c) first

 pg_upgrade -d data11/  -D  data12/  -b  /home/user/pg11/bin  -B /home/user/pg12/bin -c

if everything goes well, will get this message - 

*Clusters are compatible*

"./pg_ctl" -w -D "data12" -o "" -m smart stop >> "pg_upgrade_server.log" 2>&1

c)Feel free to report the issue to  pgsql-bugs@postgresql.org with log files.