Postgres unable to start with shmget. SHMMAX too small

Error Starting PostgreSQL, Could not create shared memory segment errors

If you have a large PostgreSQL configuration that supports many concurrent requests, you may find yourself starting it as you have increased the shared_buffers parameter or max_connections parameter, and it fails to start.

The error message will say something like could not create shared memory segment, Failed system call was shmget. This error usually means that PostgreSQL's request for a shared memory segment exceeded your kernel's SHMMAX parameter. You can either reduce the request size or reconfigure the kernel with larger SHMMAX. SHMMAX too small.

Here is the exact error message I got:

debian-testing:~# /etc/init.d/postgresql-8.3 start
Starting PostgreSQL 8.3 database server: mainThe PostgreSQL server failed to start. Please check the log output: 2010-11-17 13:53:34 EST FATAL: could not create shared memory segment: Invalid argument 2010-11-17 13:53:34 EST DETAIL: Failed system call was shmget(key=5432001, size=147382272, 03600). 2010-11-17 13:53:34 EST HINT: This error usually means that PostgreSQL's request for a shared memory segment exceeded your kernel's SHMMAX parameter. You can either reduce the request size or reconfigure the kernel with larger SHMMAX. To reduce the request size (currently 147382272 bytes), reduce PostgreSQL's shared_buffers parameter (currently 16384) and/or its max_connections parameter (currently 503). If the request size is already small, it's possible that it is less than your kernel's SHMMIN parameter, in which case raising the request size or reconfiguring SHMMIN is called for. The PostgreSQL documentation contains more information about shared memory configuration. failed!
 failed!

PostgreSQL Debian increase SHMMAX

In order to increase the maximum amount of shared memory a linux program can access, we need to adjust kernel parameters. Since we are using Debian they are available in /etc/sysctl.conf. This should be similar for many linux distributions.

We simple edit this file:

sudo vim /etc/sysctl.conf

or if you have GNOME running

sudo gedit /etc/sysctl.conf

And either search for the line or add the line (most likely will have to add) :

kernel.shmmax = 147382272

Note the number here is the same as "size" in this system call, shown in my error message above: shmget(key=5432001, size=147382272, 03600)

Now we have to put these settings into effect by running the command

sysctl -p

And now we start PostgreSQL up without the error:

debian-testing:~# /etc/init.d/postgresql-8.3 start
Starting PostgreSQL 8.3 database server: main.
debian-testing:~#

Load Balancing Web Servers using ldirectord

Web Server Load Balancing

Summary

Using ldirectord to set up load balancing in apache, using debian linux as my distro, should also work on ubuntu or any other distro for that matter (bsd, red hat, etc). See below for links to the documentation. This setup is done using ldirectord's masq mode, which is basically ip masquerading function using iptables.

Let's Begin

Im just going to take a few mins to explain my load balancing set up. What we have is 1 router, the default gateway, connected to the internet, and also connecting to an internal network. Standard set up really.

What we want to do is set up load balancing with 2 (or more, our case we have 4) apache servers.

First step - download ldirectord

http://horms.net/projects/ldirectord/ (ldirectord) is a daemon that will route traffic to a certain host based on whatever load balancing algorithm you choose. Some examples of algorithms are round robin, where it selects the next host every time, or least connections, route to the host with the least number of open connections. There are a few other options that are covered in the ldirectord documentation (man page) and specifically for the algorithms is in the IPVSADM man page under -s (scheduler) located here (http://linux.die.net/man/8/ipvsadm)

Note that ldirectord is now a sub project of linux HA project called Heartbeat. It is available as a part of that package available here http://www.linux-ha.org/wiki/Main_Page

The reason for this is ldirectord is used as a part of that package. It is also useable on its own as I will show you.

Ldirectord set up

What we have to do next is set up ldirectord after we manage to get it compiled. Here is an example of a /etc/ha.d/ldirectord.cf:

# Global Directives
checktimeout=10
checkinterval=2
autoreload=no
logfile="local0"
quiescent=yes

# Virtual Server for HTTP
virtual=10.1.1.10:80
        real=10.1.1.21:80 masq
        real=10.1.1.22:80 masq
        real=10.1.1.23:80 masq
        real=10.1.1.12:80 masq
        service=http
        request="ldirectord.html"
        receive="Test Page"
        scheduler=wlc
        protocol=tcp
        checktype=negotiate

As you can see, we have set this ldirectord program up on the host located at 10.1.1.10 which is a physical machine. Using Heartbeat you can set up a virtual IP address and have 2 or more machines take over that address when the main one goes down, but that is for another article. We are strictly focusing on setting up ldirectord here.

Setting up the load balanced machines

On each of the load balanced machines (10.1.1.21, 22, 23, and 12) we need to set the default gateway to 10.1.1.10 (the ip address of the load balancer) Please adjust accordingly to your network. This can be done using the following command on debian linux and most other types

route add default gw 10.1.1.10

Next we have set up our ldirectord to check each machine by loading the page "ldirectord.html" shown above. So really it will be trying to access the page http://10.1.1.21/ldirectord.html. We have set up a regular expression as well above saying "Test Page". As long as this url returns this string somewhere, the host will be considered "up" by ldirectord. After we have set everything up, it should be good to go.

Running ldirectord

Please make sure your config file is in /etc/ha.d/ldirectord.cf because the script will be searching there for it. Next run ldirectord by typing

/usr/bin/perl /usr/sbin/ldirectord start

You should make a init.d script for this. After you have run ldirectord, typing

ipvsadm -L

should give you some information on its status.

s5:/etc/ha.d# ipvsadm -L
IP Virtual Server version 1.2.1 (size=4096)
Prot LocalAddress:Port Scheduler Flags
  -> RemoteAddress:Port           Forward Weight ActiveConn InActConn
TCP  s5.np.local:www wlc
  -> s4.np.local:www              Masq    1      6          2245      
  -> s7.np.local:www              Masq    1      5          2275      
  -> s3.np.local:www              Masq    1      4          2563      
  -> s9.np.local:www              Masq    1      6          2074

Here it shows that all hosts are up (their "Weight" is each 1) and have active connections running to them.

Setting up external routing

Now just have your router point all web traffic to 10.1.1.10:80 and you should have full load balancing. Note there are other problems such as how are you going to make sure they are all synchronized. You can use tools such as rsync, or use NFS to mount a share that contains all your files on each of the slave servers (our approach). The NFS approach seems to work nicely, however if you are hosting webservers for file transfers it may become a bottleneck so make sure your internet network is fast (1000 mbit) and tweaking NFS settings to your liking.

Thank you, your comments are welcome!