科学地增加postgresql最大连接数

PG配置文件路径

/etc/postgresql/9.3/main/postgresql.conf

首先如何查看最大连接数

This SQL will help you

select max_conn,used,res_for_super,max_conn-used-res_for_super res_for_normal 
from 
  (select count(*) used from pg_stat_activity) t1,
  (select setting::int res_for_super from pg_settings where name=$$superuser_reserved_connections$$) t2,
  (select setting::int max_conn from pg_settings where name=$$max_connections$$) t3

Result:

max_conn | used | res_for_super | res_for_normal 
---------+------+---------------+----------------
  100    |    2 |             3 |             95
(1 row)

You can put this in shell:

#!/bin/bash
for (( c=1; c<=3600; c++ ))
do
     gsql -U pgdba -W pgdba -p 6432 -c "sql" >> /home/pgdba/res_data.log
     sleep 1  # once per second
done

or you can record the results into a table, then execute

postgres=# copy restbl to '/home/pgdba/res.csv' csv header;

to get result csv file.

其次如何科学设置

https://stackoverflow.com/a/32584211/2803340

Just increasing max_connections is bad idea. You need to increase shared_buffers and kernel.shmmax as well.

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Considerations

max_connections determines the maximum number of concurrent connections to the database server. The default is typically 100 connections.

Before increasing your connection count you might need to scale up your deployment. But before that, you should consider whether you really need an increased connection limit.

Each PostgreSQL connection consumes RAM for managing the connection or the client using it. The more connections you have, the more RAM you will be using that could instead be used to run the database.

A well-written app typically doesn't need a large number of connections. If you have an app that does need a large number of connections then consider using a tool such as pg_bouncer which can pool connections for you. As each connection consumes RAM, you should be looking to minimize their use.

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How to increase max connections

1. Increase max_connection and shared_buffers

in /var/lib/pgsql/data/postgresql.conf

change

max_connections = 100
shared_buffers = 24MB

to

max_connections = 300
shared_buffers = 80MB

The shared_buffers configuration parameter determines how much memory is dedicated to PostgreSQL to use for caching data.

• If you have a system with 1GB or more of RAM, a reasonable starting
  value for shared_buffers is 1/4 of the memory in your system.
• it's unlikely you'll find using more than 40% of RAM to work better
  than a smaller amount (like 25%)
• Be aware that if your system or PostgreSQL build is 32-bit, it might
  not be practical to set shared_buffers above 2 ~ 2.5GB.
• Note that on Windows, large values for shared_buffers aren't as
  effective, and you may find better results keeping it relatively low
  and using the OS cache more instead. On Windows the useful range is
  64MB to 512MB.

2. Change kernel.shmmax

You would need to increase kernel max segment size to be slightly larger
than the shared_buffers.

In file /etc/sysctl.conf set the parameter as shown below. It will take effect when postgresql reboots (The following line makes the kernel max to 96Mb)

kernel.shmmax=100663296

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References

How to increase max connections in postgres

Postgres Max Connections And Shared Buffers

Tuning Your PostgreSQL Server