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分类: Mysql/postgreSQL

2008-04-21 17:35:58

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Mysql Cluster: Two webserver setup (three servers required for true redundancy)
HOWTO set up a mysql cluster for two servers
Introduction

This HOWTO was designed for a classic setup of two servers behind a loadbalancer. The aim is to have true redundancy - either server can be unplugged and yet the site will remain up.
Notes

You MUST have a third server as a managment node but this can be shut down after the cluster starts. Also note that I do not recommend shutting down the managment server (see the extra notes at the bottom of this document for more information). You can not run a mysql cluster with just two servers And have true redundancy.

Although it is possible to set the cluster up on two physical servers you WILL NOT GET the ability to "kill" one server and for the cluster to continue as normal. For this you need a third server running the managment node.

I am going to talk about three servers,

mysql1.domain.com 192.168.0.1
mysql2.domain.com 192.168.0.2
mysql3.domain.com 192.168.0.3

Servers 1 and 2 will be the two that end up "clustered". This would be perfect for two servers behind a loadbalancer or using round robin DNS and is a good replacement for replication. Server 3 needs to have only minor changes made to it and does NOT require a mysql install. It can be a low-end machine and can be carrying out other tasks.

STAGE 1: Install mysql on the first two servers:

Complete the following steps on both mysql1 and mysql2:

cd /usr/local/
http://dev.mysql.com/get/Downloa ... ux-gnu-i686.tar.gz/
from/
groupadd mysql
useradd -g mysql mysql
tar -zxvf mysql-max-4.1.9-pc-linux-gnu-i686.tar.gz
rm mysql-max-4.1.9-pc-linux-gnu-i686.tar.gz
ln -s mysql-max-4.1.9-pc-linux-gnu-i686 mysql
cd mysql
scripts/mysql_install_db --user=mysql
chown -R root  .
chown -R mysql data
chgrp -R mysql .
cp support-files/mysql.server /etc/rc.d/init.d/
chmod +x /etc/rc.d/init.d/mysql.server
chkconfig --add mysql.server

Do not start mysql yet.
STAGE 2: Install and configure the managment server

You need the following files from the bin/ of the mysql directory: ndb_mgm and ndb_mgmd. Download the whole mysql-max tarball and extract them from the bin/ directory.

mkdir /usr/src/mysql-mgm
cd /usr/src/mysql-mgm
http://dev.mysql.com/get/Downloa ... ux-gnu-i686.tar.gz/
from/
tar -zxvf mysql-max-4.1.9-pc-linux-gnu-i686.tar.gz
rm mysql-max-4.1.9-pc-linux-gnu-i686.tar.gz
cd mysql-max-4.1.9-pc-linux-gnu-i686
mv bin/ndb_mgm .
mv bin/ndb_mgmd .
chmod +x ndb_mg*
mv ndb_mg* /usr/bin/
cd
rm -rf /usr/src/mysql-mgm

You now need to set up the config file for this managment:

mkdir /var/lib/mysql-cluster
cd /var/lib/mysql-cluster
vi [or emacs or any other editor] config.ini

Now, insert the following (changing the bits as indicated):

[NDBD DEFAULT]
NoOfReplicas=2
[MYSQLD DEFAULT]
[NDB_MGMD DEFAULT]
[TCP DEFAULT]
# Managment Server
[NDB_MGMD]
HostName=192.168.0.3# the IP of THIS SERVER
# Storage Engines
[NDBD]
HostName=192.168.0.1# the IP of the FIRST SERVER
DataDir= /var/lib/mysql-cluster
[NDBD]
HostName=192.168.0.2# the IP of the SECOND SERVER
DataDir=/var/lib/mysql-cluster
# 2 MySQL Clients
# I personally leave this blank to allow rapid changes of the mysql clients;
# you can enter the hostnames of the above two servers here. I suggest you dont.
[MYSQLD]
[MYSQLD]

Now, start the managment server:

ndb_mgmd

This is the mysql managment server, not maganment console. You should therefore not expect any output (we will start the console later).
STAGE 3: Configure the storage/SQL servers and start mysql

On each of the two storage/SQL servers (192.168.0.1 and 192.168.0.2) enter the following (changing the bits as appropriate):

vi /etc/my.cnf

Enter i to go to insert mode again and insert this on both servers (changing the IP address to the IP of the managment server that you set up in stage 2):

[mysqld]
ndbcluster
ndb-connectstring=192.168.0.3# the IP of the MANAGMENT (THIRD) SERVER
[mysql_cluster]
ndb-connectstring=192.168.0.3# the IP of the MANAGMENT (THIRD) SERVER

Now, we make the data directory and start the storage engine:

mkdir /var/lib/mysql-cluster
cd /var/lib/mysql-cluster
/usr/local/mysql/bin/ndbd --initial
/etc/rc.d/init.d/mysql.server start

If you have done one server now go back to the start of stage 3 and repeat exactly the same procedure on the second server.
NOTE that you should ONLY use --initial if you are either starting from scratch or have changed the config.ini file on the managment.
STAGE 4: Check its working

You can now return to the managment server (mysql3) and enter the managment console:

/usr/local/mysql/bin/ndb_mgm

Enter the command SHOW to see what is going on. A sample output looks like this:

[root@mysql3 mysql-cluster]# /usr/local/mysql/bin/ndb_mgm
-- NDB Cluster -- Management Client --
ndb_mgm>; show
Connected to Management Server at: localhost:1186
Cluster Configuration
---------------------
[ndbd(NDB)]     2 node(s)
id=2    @192.168.0.1  (Version: 4.1.9, Nodegroup: 0, Master)
id=3    @192.168.0.2  (Version: 4.1.9, Nodegroup: 0)

[ndb_mgmd(MGM)] 1 node(s)
id=1    @192.168.0.3  (Version: 4.1.9)

[mysqld(API)]   2 node(s)
id=4   (Version: 4.1.9)
id=5   (Version: 4.1.9)

ndb_mgm>;

If you see

not connected, accepting connect from 192.168.0.[1/2/3]

in the first or last two lines they you have a problem. Please email me with as much detail as you can give and I can try to find out where you have gone wrong and change this HOWTO to fix it.

If you are OK to here it is time to test mysql. On either server mysql1 or mysql2 enter the following commands: Note that we have no root password yet.

mysql
use test;
CREATE TABLE ctest (i INT) ENGINE=NDBCLUSTER;
INSERT INTO ctest () VALUES (1);
SELECT * FROM ctest;

You should see 1 row returned (with the value 1).

If this works, now go to the other server and run the same SELECT and see what you get. Insert from that host and go back to host 1 and see if it works. If it works then congratulations.

The final test is to kill one server to see what happens. If you have physical access to the machine simply unplug its network cable and see if the other server keeps on going fine (try the SELECT query). If you dont have physical access do the following:

ps aux | grep ndbd

You get an output like this:

root      5578  0.0  0.3  6220 1964 ?        S    03:14   0:00 ndbd
root      5579  0.0 20.4 492072 102828 ?     R    03:14   0:04 ndbd
root     23532  0.0  0.1  3680  684 pts/1    S    07:59   0:00 grep ndbd

In this case ignore the command "grep ndbd" (the last line) but kill the first two processes by issuing the command kill -9 pid pid:

kill -9 5578 5579

Then try the select on the other server. While you are at it run a SHOW command on the managment node to see that the server has died. To restart it, just issue

ndbd

NOTE no --inital!
Further notes about setup

I strongly recommend that you read all of this (and bookmark this page). It will almost certainly save you a lot of searching.
The Managment Server

I strongly recommend that you do not stop the managment server once it has started. This is for several resons:
The server takes hardly any server resources
If a cluster falls over, you want to be able to just ssh in and type ndbd to stat it. You dont want to have to start messing around with another server
If you want to take backups then you need the managment server up
The cluster log is sent to the management server so to check what is going on in the cluster or has happened since last this is an important tool
All commands from the ndb_mgm client is sent to the management server and thus no management commands without management server.
The managment server is required in case of cluster reconfiguration (crashed server or network split). In the case that it is not running, "split-brain" scenario will occure. The management server arbitration role is required for this type of setup to provide better fault tollerance.

However you are welcome to stop the server if you prefer.
Starting and stopping ndbd automatically on boot

To achieve this, do the following on both mysql1 and mysql2:

echo "ndbd" >; /etc/rc.d/init.d/ndbd
chmod +x /etc/rc.d/init.d/ndbd
chkconfig --add ndbd

Note that this is a really quick script. You ought really to write one that at least checks if ndbd is already started on the machine.
Use of hostnames

You will note that I have used IP addresses exclusively throught this setup. This is because using hostnames simply increases the number of things that can go wrong. Mikael Ronstr
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