This quick start tutorial is based on CentOS 7.2, using the
The cluster we are about to build includes three servers called
srv3. Each of them have two network interfaces
eth1. IP addresses of these servers are
192.168.122.5x/24 on the first
192.168.123.5x/24 on the second one.
The IP address
pgsql-vip in this tutorial, will be
set on the server hosting the master PostgreSQL instance.
Considering the firewall, we have to allow the network traffic related to the cluster and PostgreSQL to go through:
firewall-cmd --permanent --add-service=high-availability firewall-cmd --add-service=high-availability firewall-cmd --permanent --add-service=postgresql firewall-cmd --add-service=postgresql
During the cluster setup, we use the node names in various places,
make sure all your server hostnames can be resolved to the correct IPs. We
usually set this in the
192.168.122.50 pgsql-vip 192.168.122.51 srv1 192.168.122.52 srv2 192.168.122.53 srv3 192.168.123.51 srv1-alt 192.168.123.52 srv2-alt 192.168.123.53 srv3-alt
Now, the three servers should be able to ping each others, eg.:
root@srv1:~# for s in srv1 srv2 srv3; do ping -W1 -c1 $s; done| grep icmp_seq 64 bytes from srv1 (192.168.122.51): icmp_seq=1 ttl=64 time=0.028 ms 64 bytes from srv2 (192.168.122.52): icmp_seq=1 ttl=64 time=0.296 ms 64 bytes from srv3 (192.168.122.53): icmp_seq=1 ttl=64 time=0.351 ms
Run this whole chapter on ALL nodes.
We are using the PostgreSQL packages from the PGDG repository. Here is how to install and set up this repository on your system:
yum install -y http://yum.postgresql.org/9.6/redhat/rhel-7-x86_64/pgdg-centos96-9.6-3.noarch.rpm
Make sure to double adapt the previous command with the latest package available and the PostgreSQL version you need.
We can now install everything we need for our cluster:
yum install -y pacemaker postgresql96 postgresql96-contrib postgresql96-server resource-agents pcs fence-agents-all fence-agents-virsh
Finally, we need to install the “PostgreSQL Automatic Failover” (PAF) resource agent:
yum install -y https://github.com/dalibo/PAF/releases/download/v2.1.0/resource-agents-paf-2.1.0-1.noarch.rpm
WARNING: building PostgreSQL standby is not the main subject here. The following steps are **quick and dirty, VERY DIRTY**. They lack of security, WAL retention and so on. Rely on the PostgreSQL documentation for a proper setup.
The resource agent requires the PostgreSQL instances to be already set up, ready to start and slaves ready to replicate. Make sure to setup your PostgreSQL master on your preferred node to host the master: during the very first startup of the cluster, PAF detects the master based on its shutdown status.
Moreover, it requires a
recovery.conf template ready to use. You can create
recovery.conf file suitable to your needs, the only requirements are:
standby_mode = on
recovery_target_timeline = 'latest'
application_nameset to the node name
Last but not least, make sure each instance will not be able to replicate with
itself! A scenario exists where the master IP address
pgsql-vip will be on
the same node than a standby for a very short lap of time!
pg_hba.conffiles are different on each node, it is best to keep them out of the
$PGDATAso you do not have to deal with them (or worst: forget to edit them) each time you rebuild a standby! We advice you to deal with this using the
hba_fileparameter in your
recovery_templateparameter in PAF for the
Here are some quick steps to build your primary PostgreSQL instance and its
standbys. This quick start considers
srv1 is the preferred master.
On the primary:
/usr/pgsql-9.6/bin/postgresql96-setup initdb su - postgres cd 9.6/data/ cat <<EOP >> postgresql.conf listen_addresses = '*' wal_level = replica max_wal_senders = 10 hot_standby = on hot_standby_feedback = on EOP cat <<EOP >> pg_hba.conf # forbid self-replication host replication postgres 192.168.122.50/32 reject host replication postgres $(hostname -s) reject # allow any standby connection host replication postgres 0.0.0.0/0 trust EOP cat <<EOP > recovery.conf.pcmk standby_mode = on primary_conninfo = 'host=192.168.122.50 application_name=$(hostname -s)' recovery_target_timeline = 'latest' EOP exit systemctl start postgresql-9.6 ip addr add 192.168.122.50/24 dev eth0
Now, on each standby, clone the primary. E.g.:
su - postgres pg_basebackup -h pgsql-vip -D ~postgres/9.6/data/ -X stream -P cd ~postgres/9.6/data/ sed -ri s/srv[0-9]+/$(hostname -s)/ pg_hba.conf sed -ri s/srv[0-9]+/$(hostname -s)/ recovery.conf.pcmk cp recovery.conf.pcmk recovery.conf exit systemctl start postgresql-9.6
Finally, make sure to stop the PostgreSQL services everywhere and to disable them, as Pacemaker will take care of starting/stopping everything for you during cluster normal cluster operations:
systemctl stop postgresql-9.6 systemctl disable postgresql-9.6
And remove the master IP address from
ip addr del 192.168.122.50/24 dev eth0
It is advised to keep Pacemaker off on server boot. It helps the administrator to investigate after a node fencing before Pacemaker starts and potentially enters in a death match with the other nodes. Make sure to disable Corosync as well to avoid unexpected behaviors. Run this on all nodes:
systemctl disable corosync # important! systemctl disable pacemaker
This guide uses the cluster management tool
pcsd provided by RHEL to ease the
creation and setup of a cluster. It allows to create the cluster from command
line, without editing configuration files or XML by hands.
pcsd uses the hacluster system user to work and communicate with other
members of the cluster. We need to set a password to this user so it can
authenticate to other nodes easily. As cluster management commands can be run on
any member of the cluster, it is recommended to set the same password everywhere
to avoid confusions:
Enable and start the
pcsd daemon on all the nodes:
systemctl enable pcsd systemctl start pcsd
Now, authenticate each node to the other ones using the following command:
pcs cluster auth srv1 srv2 srv3 -u hacluster
pcs cli tool is able to create and start the whole cluster for us. From
one of the nodes, run the following command:
pcs cluster setup --name cluster_pgsql srv1,srv1-alt srv2,srv2-alt srv3,srv3-alt
NOTE: If you don’t have an alternative network available use the following syntax instead of the previous one:
pcs cluster setup --name cluster_pgsql srv1 srv2 srv3
Make sure you have a redundant network at system level. This is a **CRITICAL** part of your cluster.
This command creates the
/etc/corosync/corosync.conf file and propagate
it everywhere. For more information about it, read the
WARNING: whatever you edit in your
/etc/corosync/corosync.conffile, **ALWAYS** make sure all the nodes in your cluster has the exact same copy of the file.
You can now start the whole cluster from one node:
pcs cluster start --all
After some seconds of startup and cluster membership stuff, you should be able
to see your three nodes up in
root@srv1:~# crm_mon -n1D Node srv1: online Node srv2: online Node srv3: online
Now the cluster run, let’s start with some basic setup of the cluster. Run the following command from one node only (the cluster takes care of broadcasting the configuration on all nodes):
pcs -f cluster1.xml resource defaults migration-threshold=5 pcs -f cluster1.xml resource defaults resource-stickiness=10
This sets two default values for resources we create in the next chapter:
resource-stickiness: adds a sticky score for the resource on its current node. It helps avoiding a resource move back and forth between nodes where it has the same score.
migration-threshold: this controls how many time the cluster tries to recover a resource on the same node before moving it on another one.
One of the most important resource in your cluster is the one able to fence a
node. Please, stop reading this quick start and read our fencing
documentation page before building your cluster. Take a deep breath, and open
docs/FENCING.md in the source code of PAF or read online:
WARNING: I really mean it. You need fencing. PAF is expecting fencing to work in your cluster. Without fencing, you will experience cluster refusing to move anything, even with stonith disabled, or worst, a split brain if you bend it hard enough to make it work anyway. If you don’t mind taking time rebuilding a database with corrupt and/or incoherent data and constraints, that’s fine though.
In this tutorial, we choose to create one fencing resource per node to fence.
They are called
fence_vm_xxxand use the fencing agent
to power on or off a virtual machine using the
virsh command through a ssh
connexion to the hypervisor.
WARNING: unless you build your PoC cluster using libvirt for VM management, there’s great chances you will need to use a different STONITH agent. The stonith setup is provided as a simple example, be prepared to adjust it.
Now you’ve been warned again and again, let’s populating the cluster with some
sample STONITH resources using virsh over ssh. First, we need to allow ssh
password-less authentication to
<user>@192.168.122.1 so these fencing
resource can work. Again, this is specific to this setup. Depending on your
fencing topology, you might not need this step. Run on all node:
Check the ssh connections are working as expected.
We can now create one STONITH resource for each node. Each fencing resource will not be allowed to run on the node it is supposed to fence:
pcs cluster cib cluster1.xml pcs -f cluster1.xml stonith create fence_vm_srv1 fence_virsh \ pcmk_host_check="static-list" pcmk_host_list="srv1" \ ipaddr="192.168.122.1" login="<user>" port="srv1-c7" \ action="off" identity_file="/root/.ssh/id_rsa" pcs -f cluster1.xml stonith create fence_vm_srv2 fence_virsh \ pcmk_host_check="static-list" pcmk_host_list="srv2" \ ipaddr="192.168.122.1" login="<user>" port="srv2-c7" \ action="off" identity_file="/root/.ssh/id_rsa" pcs -f cluster1.xml stonith create fence_vm_srv3 fence_virsh \ pcmk_host_check="static-list" pcmk_host_list="srv3" \ ipaddr="192.168.122.1" login="<user>" port="srv3-c7" \ action="off" identity_file="/root/.ssh/id_rsa" pcs -f cluster1.xml constraint location fence_vm_srv1 avoids srv1=INFINITY pcs -f cluster1.xml constraint location fence_vm_srv2 avoids srv2=INFINITY pcs -f cluster1.xml constraint location fence_vm_srv3 avoids srv3=INFINITY pcs cluster cib-push cluster1.xml
crm_mon You should see the three resources appearing in your cluster
and being dispatched on nodes.
In this last chapter we create three resources:
pgsqld defines the properties of a PostgreSQL instance: where it is
located, where are its binaries, its configuration files, how to montor it, and
pgsql-ha resource controls all the PostgreSQL instances
pgsqld in your
cluster, decides where the primary is promoted and where the standbys
pgsql-master-ip resource controls the
pgsql-vip IP address. It is
started on the node hosting the PostgreSQL master resource.
Now the fencing is working, we can add all other resources and constraints all together in the same time. Create a new offline CIB:
pcs cluster cib cluster1.xml
We add the PostgreSQL
pgsqld resource and the multistate
responsible to clone it everywhere and define the roles (master/slave) of each
# pgsqld pcs -f cluster1.xml resource create pgsqld ocf:heartbeat:pgsqlms \ bindir=/usr/pgsql-9.6/bin pgdata=/var/lib/pgsql/9.6/data \ op start timeout=60s \ op stop timeout=60s \ op promote timeout=30s \ op demote timeout=120s \ op monitor interval=15s timeout=10s role="Master" \ op monitor interval=16s timeout=10s role="Slave" \ op notify timeout=60s # pgsql-ha pcs -f cluster1.xml resource master pgsql-ha pgsqld notify=true
Note that the values for
interval on each operation are based
on the minimum suggested value for PAF Resource Agent. These values should be
adapted depending on the context.
We add the IP address which should be started on the primary node:
pcs -f cluster1.xml resource create pgsql-master-ip ocf:heartbeat:IPaddr2 \ ip=192.168.122.50 cidr_netmask=24 op monitor interval=10s
We now define the collocation between
start/stop and promote/demote order for these resources must be asymetrical: we
MUST keep the master IP on the master during its demote process so the
standbies receive everything during the master shutdown.
pcs -f cluster1.xml constraint colocation add pgsql-master-ip with master pgsql-ha INFINITY pcs -f cluster1.xml constraint order promote pgsql-ha then start pgsql-master-ip symmetrical=false kind=Mandatory pcs -f cluster1.xml constraint order demote pgsql-ha then stop pgsql-master-ip symmetrical=false kind=Mandatory
We can now push our CIB to the cluster, which will start all the magic stuff:
pcs cluster cib-push cluster1.xml