Thread: Multiple PostgreSQL instances on one machine

I am trying to consolidate some machines in my server room particularly in
the testing environment and I was hoping someone could point me in the
right direction.

I currently have three machines running PostgreSQL for testing purposes.
Each week a backup is made of the production database and this is deployed
onto these three machines. None of these machines is any where near
capacity. I would like to host all three instances on one machine.

I know that "initdb" can be used to create additional installations, but
the part I am unsure about is the tablespace. The production database has
a tablespace defined that resides on its SSD. When I use the production
backup, I have to create a mount point on the test machine with the same
name as the one on the production database. I am not certain how I would
handle this situation if I am serving three separate instances of that
database from one computer.

On 06/08/2018 01:29 PM, Tony Sullivan wrote:
> I am trying to consolidate some machines in my server room particularly in
> the testing environment and I was hoping someone could point me in the
> right direction.
> 
> I currently have three machines running PostgreSQL for testing purposes.
> Each week a backup is made of the production database and this is deployed
> onto these three machines. None of these machines is any where near
> capacity. I would like to host all three instances on one machine.
> 
> I know that "initdb" can be used to create additional installations, but
> the part I am unsure about is the tablespace. The production database has
> a tablespace defined that resides on its SSD. When I use the production
> backup, I have to create a mount point on the test machine with the same
> name as the one on the production database. I am not certain how I would
> handle this situation if I am serving three separate instances of that
> database from one computer.
> 

 From here:

https://www.postgresql.org/docs/10/static/manage-ag-tablespaces.html

"The directory $PGDATA/pg_tblspc contains symbolic links that point to 
each of the non-built-in tablespaces defined in the cluster. Although 
not recommended, it is possible to adjust the tablespace layout by hand 
by redefining these links. Under no circumstances perform this operation 
while the server is running. Note that in PostgreSQL 9.1 and earlier you 
will also need to update the pg_tablespace catalog with the new 
locations. (If you do not, pg_dump will continue to output the old 
tablespace locations.)"

A quick test here showed that you can create a new directory and move 
the contents of the existing tablespace into it and then relink the 
tablespace to the new location.

create tablespace tblspc_test location '/home/postgres/test_tblspc';

create table tblspc_table(id int) tablespace tblspc_test ;


select * from pg_class where relname = 'tblspc_table';
-[ RECORD 1 ]-------+-------------
relname             | tblspc_table
relnamespace        | 2200
reltype             | 1836557
reloftype           | 0
relowner            | 10
relam               | 0
relfilenode         | 1836555
reltablespace       | 1836554



So in $DATA/pg_tblspc:

1836554 -> /home/postgres/test_tblspc/


mkdir /home/postgres/tblspc_1

cd /home/postgres/test_tblspc/

cp -r PG_10_201707211/ ../tblspc_1/

shutdown Postgres

cd $DATA/pg_tblspc

rm 1836554

ln -s /home/postgres/tblspc_1 1836554

start Postgres

select * from pg_class where relname = 'tblspc_table';
-[ RECORD 1 ]-------+-------------
relname             | tblspc_table
relnamespace        | 2200
reltype             | 1836557
reloftype           | 0
relowner            | 10
relam               | 0
relfilenode         | 1836555
reltablespace       | 1836554


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On Fri, Jun 8, 2018 at 3:29 PM Tony Sullivan <tony@exquisiteimages.com> wrote:
>
> I am trying to consolidate some machines in my server room particularly in
> the testing environment and I was hoping someone could point me in the
> right direction.
>
> I currently have three machines running PostgreSQL for testing purposes.
> Each week a backup is made of the production database and this is deployed
> onto these three machines. None of these machines is any where near
> capacity. I would like to host all three instances on one machine.
>
> I know that "initdb" can be used to create additional installations, but
> the part I am unsure about is the tablespace. The production database has
> a tablespace defined that resides on its SSD. When I use the production
> backup, I have to create a mount point on the test machine with the same
> name as the one on the production database. I am not certain how I would
> handle this situation if I am serving three separate instances of that
> database from one computer.

Generally speaking, this not an issue.  You can consolidate all three
machines to one cluster, or to multiple clusters running on same box.
The approaches have pros and cons relative to each other.  I would
personally tend to simple create different databases on the same
cluster unless you had a reason not to.  The database would share:

*) database roles (in particular superuser accounts)
*) WAL (possibly presenting a i/o bottleneck)
*) replication for HS/SR (which is all or nothing at the cluster level)
*) shared memory
*) availability -- if one database crashes they will all crash
*) tcp/ip port (this is good -- you don't have to worry about what
port you're connecting to)

merlin