High-Availibility for Postgres, based on Pacemaker and Corosync.
This quick start purpose is to help you to build your first cluster to experiment with. It does not implement various good practices related to your system, Pacemaker or PostgreSQL. This quick start alone is not enough. During your journey in building a safe HA cluster, you must train about security, network, PostgreSQL, Pacemaker, PAF, etc. In regard with PAF, make sure to read carefully documentation from https://clusterlabs.github.io/PAF/documentation.html.
This tutorial is based on CentOS 7.2, using Pacemaker 1.1, the pcs command
and PostgreSQL 9.6.
Table of contents:
To install PAF and PostgreSQL, this tutorial uses the PGDG repository maintained by the PostgreSQL community. Here is how to add it:
yum install -y https://yum.postgresql.org/reporpms/EL-7-x86_64/pgdg-redhat-repo-latest.noarch.rpm
NOTE: you can install PostgreSQL from official CentOS repository if you prefer.
The cluster we are about to build includes three servers called srv1,
srv2 and srv3. IP addresses of these servers are 192.168.122.5x/24.
NOTE: It is essential to setup network redundancy, either at system level using eg. bonding or teaming, or at cluster level.
The IP address 192.168.122.50, called pgsql-vip in this tutorial, will be
set on the server hosting the primary PostgreSQL instance.
If the firewall is enabled, 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 /etc/hosts file:
192.168.122.50 pgsql-vip
192.168.122.51 srv1
192.168.122.52 srv2
192.168.122.53 srv3
Now, the three servers should be able to ping each others, eg.:
root@srv1:~# for s in srv1 srv2 srv3; do ping -W1 -c1 $s; done| grep icmp_seq
64 bytes from srv1 (192.168.122.51): icmp_seq=1 ttl=64 time=0.028 ms
64 bytes from srv2 (192.168.122.52): icmp_seq=1 ttl=64 time=0.296 ms
64 bytes from srv3 (192.168.122.53): icmp_seq=1 ttl=64 time=0.351 ms
Make sure hostnames are correctly set on each nodes, or use hostnamectl.
Eg.:
hostnamectl set-hostname srv1
Run this whole chapter on ALL nodes.
Let’s install everything we need: PostgreSQL, Pacemaker, cluster related packages and PAF:
yum install -y postgresql96 postgresql96-contrib postgresql96-server \
pacemaker resource-agents resource-agents-paf pcs \
fence-agents-all fence-agents-virsh
WARNING: building PostgreSQL standby is not the main subject here. The following steps are **quick and dirty, VERY DIRTY**. They lack of security, WAL retention and so on. Rely on the PostgreSQL documentation for a proper setup.
The resource agent requires the PostgreSQL instances to be already set up, ready to start and standbys ready to replicate. Make sure to setup your primary on your preferred node to host it: during the very first startup of the cluster, PAF detects the primary based on its shutdown status.
Moreover, it requires a recovery.conf template ready to use. You can create a
recovery.conf file suitable to your needs, the only requirements are:
standby_mode = onrecovery_target_timeline = 'latest'primary_conninfo with an application_name set to the node nameLast but not least, make sure each instance is not able to replicate with
itself! A scenario exists where the primary IP address pgsql-vip will be on
the same node than a standby for a very short lap of time!
WARNING:
primary_conninfoandpg_hba.confare different on each node. It is best to keep them out of the$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 thehba_fileparameter in yourpostgresql.conffile andrecovery_templateparameter in PAF for therecovery.conf.pcmkfile.
Here are some quick steps to build your primary PostgreSQL instance and its
standbys. This quick start considers srv1 is the preferred primary node.
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 other nodes, clone the primary as standby and configure it:
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
Check your three instances are replicating as expected (in processes, logs,
pg_stat_replication, etc).
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. Start with the primary:
systemctl stop postgresql-9.6
systemctl disable postgresql-9.6
And remove the vIP from srv1:
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 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:
passwd hacluster
Make sure the pcsd daemon is enabled and started on all 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
The 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 srv2 srv3
NOTE: Make sure you have a redundant network at system level. This is a **CRITICAL** part of your cluster. If you have second interfaces not in bonding or teaming already (prefered method), you can add them to the cluster setup using eg.:
pcs cluster setup --name cluster_pgsql srv1,srv1-alt srv2,srv2-alt srv3,srv3-alt
This command creates the /etc/corosync/corosync.conf file and propagate
it everywhere. For more information about it, read the corosync.conf(5)
manual page.
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 usepcs cluster sync.
You can now start the whole cluster from one node:
pcs cluster start --all
After some seconds of startup and cluster membership stuffs, you should be able
to see your three nodes up in crm_mon (or pcs status):
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 resource defaults migration-threshold=5
pcs 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.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 read: http://clusterlabs.github.com/PAF/fencing.html.
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.
NOTE: if you can’t have active fencing, look as storage base death or watchdog methods. They are both described in the fencing documentation.
In this tutorial, we choose to create one fencing resource per node to fence.
They are called fence_vm_xxxand use the fencing agent fence_virsh, allowing
to power on or off a virtual machine using the virsh command through a ssh
connection 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:
ssh-keygen
ssh-copy-id <user>@192.168.122.1
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.
Note that in the port argument of the following commands, srv[1-3]-c7 are
the names of the virutal machines as known by libvirtd side. See manpage
fence_virsh(8) for more infos.
pcs cluster cib fencing.xml
pcs -f fencing.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" \
identity_file="/root/.ssh/id_rsa"
pcs -f fencing.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" \
identity_file="/root/.ssh/id_rsa"
pcs -f fencing.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" \
identity_file="/root/.ssh/id_rsa"
pcs -f fencing.xml constraint location fence_vm_srv1 avoids srv1=INFINITY
pcs -f fencing.xml constraint location fence_vm_srv2 avoids srv2=INFINITY
pcs -f fencing.xml constraint location fence_vm_srv3 avoids srv3=INFINITY
pcs cluster cib-push scope=configuration fencing.xml
Using 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, pgsql-ha,
and pgsql-pri-ip.
The pgsqld defines the properties of a PostgreSQL instance: where it is
located, where are its binaries, its configuration files, how to montor it, and
so on.
The pgsql-ha resource controls all the PostgreSQL instances pgsqld in your
cluster, decides where the primary is promoted and where the standbys
are started.
The pgsql-pri-ip resource controls the pgsql-vip IP address. It is
started on the node hosting the PostgreSQL primary 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 pgsql-ha
responsible to clone it everywhere and define the roles (Master/Slave) of
each clone:
# 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 timeout and interval on each operation are based
on the minimum suggested value for PAF Resource Agent. These values should be
adapted depending on the context.
The last line of this command declare the resource pgsqld as promotable. The
pgsqld-clone resource controls roles of pgsqld clones in the cluster.
We add the IP address which should be started on the primary node:
pcs -f cluster1.xml resource create pgsql-pri-ip ocf:heartbeat:IPaddr2 \
ip=192.168.122.50 cidr_netmask=24 op monitor interval=10s
We now define the collocation between pgsql-ha and pgsql-pri-ip. The
start/stop and promote/demote order for these resources must be asymetrical: we
MUST keep the vIP on the primary during its demote process so the
standbies receive everything during the primary shutdown.
pcs -f cluster1.xml constraint colocation add pgsql-pri-ip with master pgsql-ha INFINITY
pcs -f cluster1.xml constraint order promote pgsql-ha then start pgsql-pri-ip symmetrical=false kind=Mandatory
pcs -f cluster1.xml constraint order demote pgsql-ha then stop pgsql-pri-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 scope=configuration cluster1.xml
Now you know the basics to build a Pacemaker cluster hosting some PostgreSQL instances replicating with each others, you should probably check: