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 Debian 9.1, using Pacemaker 1.1.16 and
the crm command version 2.3.2.
Table of contents:
To install PostgreSQL and PAF, this tutorial uses the PGDG repository maintained by the PostgreSQL community (and actually Debian maintainers). Here is how to add it:
cat <<EOF >> /etc/apt/sources.list.d/pgdg.list
deb http://apt.postgresql.org/pub/repos/apt/ stretch-pgdg main
EOF
Now, update your local apt cache:
apt install ca-certificates gpg
wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add
apt update
apt install pgdg-keyring
The cluster we are about to build includes three servers called srv1,
srv2 and srv3. IP addresses of these servers are 192.168.122.6x/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 192.168.122.60, called pgsql-vip in this tutorial, will be
set on the server hosting the primary PostgreSQL instance.
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:
192.168.122.60 pgsql-vip
192.168.122.61 srv1
192.168.122.62 srv2
192.168.122.63 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 (192.168.122.61): icmp_seq=1 ttl=64 time=0.028 ms
64 bytes from srv2 (192.168.122.62): icmp_seq=1 ttl=64 time=0.296 ms
64 bytes from srv3 (192.168.122.63): 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:
apt install --no-install-recommends pacemaker fence-agents crmsh net-tools
apt install postgresql-9.6 postgresql-contrib-9.6 postgresql-client-9.6
apt install resource-agents-paf
We add the --no-install-recommends because the apt tools are setup by default
to install all recommended packages in addition to the usual dependencies. This
might be fine in most case, but we want to keep this quick start small, easy
and clear. Installing recommended packages requires some more attention on
other subjects not related to this document (eg. setting up some IPMI daemon).
By default, Debian set up the instances to put the temporary activity
statistics inside a sub folder of /var/run/postgresql/. This sub folder is
created by the debian specific tool pg_ctlcluster on instance startup.
PAF only use tools provided by the PostgreSQL projects, no other ones specifics
to some other packages or operating system. That means that this required sub
folder set up in stats_temp_directory is never created and leads to error on
instance startup by Pacemaker.
To create this sub folder on system initialization, we need to extend the
existing systemd-tmpfiles configuration for postgresql to add it. In our
environment stats_temp_directory is set
to /var/run/postgresql/9.6-main.pg_stat_tmp, so we create the following
file:
cat <<EOF > /etc/tmpfiles.d/postgresql-part.conf
# Directory for PostgreSQL temp stat files
d /var/run/postgresql/9.6-main.pg_stat_tmp 0700 postgres postgres - -
EOF
To take this file in consideration immediately without rebooting the server, run the following command:
systemd-tmpfiles --create /etc/tmpfiles.d/postgresql-part.conf
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!
NOTE: as
recovery.conf.pcmkandpg_hba.conffiles are different on each node, make sure 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! Luckily, Debian packaging already enforce this as configuration files are all located in/etc/postgresql.
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 all nodes:
su - postgres
cd /etc/postgresql/9.6/main/
cat <<EOP >> postgresql.conf
listen_addresses = '*'
wal_level = replica
max_wal_senders = 10
hot_standby = on
hot_standby_feedback = on
logging_collector = on
EOP
cat <<EOP >> pg_hba.conf
# forbid self-replication
host replication postgres 192.168.122.60/32 reject
host replication postgres $(hostname -s) reject
# allow any standby connection
host replication postgres 0.0.0.0/0 trust
EOP
cat <<EOP > recovery.conf.pcmk
standby_mode = on
primary_conninfo = 'host=192.168.122.60 application_name=$(hostname -s)'
recovery_target_timeline = 'latest'
EOP
exit
On srv1, the primary, restart the instance and give it the primary vIP address
(adapt the eth0 interface to your system):
systemctl restart postgresql@9.6-main
ip addr add 192.168.122.60/24 dev eth0
Now, on each standby (srv2 and srv3 here), we have to cleanup the instance
created by the package and clone the primary. E.g.:
systemctl stop postgresql@9.6-main
su - postgres
rm -rf 9.6/main/
pg_basebackup -h pgsql-vip -D ~postgres/9.6/main/ -X stream -P
cp /etc/postgresql/9.6/main/recovery.conf.pcmk ~postgres/9.6/main/recovery.conf
exit
systemctl start postgresql@9.6-main
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 disable --now postgresql@9.6-main
echo disabled > /etc/postgresql/9.6/main/start.conf
And remove the vIP address from srv1:
ip addr del 192.168.122.60/24 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
Moreover, on Pacemaker and Corosync installation, Debian packaging automatically creates and start a dummy isolated node. We need to move it out of our way by stopping it before creating the real one:
systemctl stop pacemaker.service corosync.service
This guide uses the cluster management tool crm 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.
crm relies heavily on SSH to transfer files between nodes and execute remote
commands. It requires the system user root to be able to connect to all
remote nodes without password. The easiest way to go is to generate a SSH keys
on each node and put the public part in the ~root/.ssh/authorized_keys file
on other nodes.
On all nodes:
ssh-keygen # do no set any password
ssh-copy-id srv1
ssh-copy-id srv2
ssh-copy-id srv3
The crm cli tool is able to create and start the whole cluster for us. From
one of the nodes, run the following command:
crm cluster init srv1 srv2 srv3
This command creates the /etc/corosync/corosync.conf file and propagate
it everywhere. This file must always be the same among all nodes. For more
information about it, read the corosync.conf(5) manual page.
Your cluster is now supposed to run, the following command should prove it, just give it some time for the node to negotiate between them:
crm status
If this command returns an error after some time, start the cluster yourself by hand running the following command on all nodes:
crm cluster start
After some seconds of startup and cluster membership stuff, you should be able
to see your three nodes up in crm_mon (or crm status):
root@srv1:~# crm_mon -n1D
Node srv1: online
Node srv2: online
Node srv3: online
WARNING: make sure you have a redundant network at system level. This is a **CRITICAL** part of your cluster.
crmdoes not support redundant rings configuration in corosync. To avoid having your network being a SPoF, either setup some redundancy on network link level (better) or edit by hands the corosync configuration to add a second ring (good enough) using the following commands:crm corosync edit crm corosync pushHere is a sample diff-formated edition of
/etc/corosync/corosync.confto add the second ring:@@ -16,10 +16,12 @@ interface { ringnumber: 0 bindnetaddr: 192.168.122.0 - mcastaddr: 239.19.196.75 - mcastport: 5405 - ttl: 1 } + interface { + ringnumber: 1 + bindnetaddr: 192.168.123.0 + } + rrp_mode: passive transport: udpu } @@ -41,16 +43,19 @@ node { ring0_addr: srv1 + ring1_addr: srv1-alt nodeid: 1 } node { ring0_addr: srv3 + ring1_addr: srv3-alt nodeid: 2 } node { ring0_addr: srv2 + ring1_addr: srv2-alt nodeid: 3 }You will need to restart the cluster after such a change. Run on all node:
crm cluster stop crm cluster start
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.
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):
crm conf <<EOC
rsc_defaults resource-stickiness=10
rsc_defaults migration-threshold=5
EOC
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]-d9 are
the names of the virutal machines as known by libvirtd side. See manpage
fence_virsh(8) for more infos.
crm conf<<EOC
primitive fence_vm_srv1 stonith:fence_virsh \
params pcmk_host_check="static-list" pcmk_host_list="srv1" \
ipaddr="192.168.122.1" login="<user>" \
identity_file="/root/.ssh/id_rsa" port="srv1-d9" \
op monitor interval=10s
primitive fence_vm_srv2 stonith:fence_virsh \
params pcmk_host_check="static-list" pcmk_host_list="srv2" \
ipaddr="192.168.122.1" login="<user>" \
identity_file="/root/.ssh/id_rsa" port="srv2-d9" \
op monitor interval=10s
primitive fence_vm_srv3 stonith:fence_virsh \
params pcmk_host_check="static-list" pcmk_host_list="srv3" \
ipaddr="192.168.122.1" login="<user>" \
identity_file="/root/.ssh/id_rsa" port="srv3-d9" \
op monitor interval=10s
location fence_vm_srv1-avoids-srv1 fence_vm_srv1 -inf: srv1
location fence_vm_srv2-avoids-srv2 fence_vm_srv2 -inf: srv2
location fence_vm_srv3-avoids-srv3 fence_vm_srv3 -inf: srv3
EOC
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:
pgsqld resourcepgsql-ha responsible to clone pgsqld everywhere and
define the roles (Master/Slave) of each clonecrm conf <<EOC
# 1. resource pgsqld
primitive pgsqld pgsqlms \
params pgdata="/var/lib/postgresql/9.6/main" \
bindir="/usr/lib/postgresql/9.6/bin" \
pghost="/var/run/postgresql" \
recovery_template="/etc/postgresql/9.6/main/recovery.conf.pcmk" \
start_opts="-c config_file=/etc/postgresql/9.6/main/postgresql.conf" \
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
# 2. resource pgsql-ha
ms pgsql-ha pgsqld meta notify=true
# 3. the vIP address
primitive pgsql-pri-ip IPaddr2 \
params ip=192.168.122.60 cidr_netmask=24 \
op monitor interval=10s
# 4. colocation of the pgsql-ha master and the vIP address
colocation ip-with-pri inf: pgsql-pri-ip pgsql-ha:Master
# 5. ordering constraint
order promote-then-ip Mandatory: \
pgsql-ha:promote pgsql-pri-ip:start \
sequential=true symmetrical=false
order demote-then-stop-ip Mandatory: \
pgsql-ha:demote pgsql-pri-ip:stop \
sequential=true symmetrical=false
EOC
In step 5, 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 its shutdown.
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.
Now you know the basics to build a Pacemaker cluster hosting some PostgreSQL instances replicating with each others, you should probably check: