V4 High Availability

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Background

SARK HA V1 and V2 used Heartbeat and our own in-house written handlers for file replication and failover. The cluster was very reliable but limited in its ability to quickly fail-back after a protracted failover event, usually involving some level of manual intervention. SARK V4 high availability uses an openAIS stack with DRBD to create a true cluster. Whereas HA1/HA2 ran as a fixed primary/secondary pair, V4 is not particularly concerned which cluster element is currently active, except in one corner case; that of switching PRI circuits between cluster nodes during fail-over and fail-back (See the section on rhino support below). The three principle software components used by V4 HA are Pacemaker, Corosync and DRBD. In broad terms, Pacemaker is the cluster manager, Corosync is the cluster communication component (it replaces Heartbeat) and DRBD (Distributed Replicated Block Device) is the data manager. DRBD is effectively a RAID 1 device which spans two or more cluster nodes. SARK, and indeed Asterisk, know little about the cluster although both components are aware that it is running.

ASHA

A raw openAIS stack install is not for the feint-hearted. If you are unfamiliar with the components, you can spend a long frustrating time playing "whack-a-mole" with recalcitrant servers that will simply refuse to do as they're asked. You must also define a strategy for addressing application data, which is now in the DRBD partition rather than where it would normally reside. For SARK, we built a little helper utility called ASHA (Asterisk SARK High Availability) which reduces the entire install to 2 commands. ASHA is a work in progress but we published it early in the hope that it may be of use. It is not a panacea and it is by no means perfect (the idempotency is suspect in a few areas) but it will get you a vanilla SARK cluster up and running in just a few minutes... which brings us to platform choices;

Platform

V4 HA runs on Debian Wheezy. It cannot currently run on SME Server (SARK's traditional home). This is not due to any architectural limitation of V4 HA but rather the extreme difficulty of installing the openAIS stack on SME Server 8.x. It is unlikely that this will change in the foreseeable future.

HA V3 vs HA V4

  • No clean-up required after a V4 fail-over/fail-back
  • V4 nodes run full firewall (V3 nodes require an upstream firewall)
  • V4 nodes run Fail2ban (V3 runs ossec)
  • V4 fail-over is slightly slower than V3 (around 30 seconds vs 12 seconds in V3)
  • V4 Node removal/replacement is much easier – nodes can be pre-prepared and sync is fully automated
  • V4 clusters can detect and resolve Asterisk hangs and freezes
  • V4 clusters need to be pre-planned during initial Linux install
  • V3-HA upgrade to V4-HA requires a re-install
  • V4 upgrade to V4-HA requires either a re-install or repartitioning (or an extra drive)
  • V4 clusters can run at dynamic Eth0 IP addresses
  • Rhino card support is much simplified in V4
  • V4 Nodes require a minimum of 2 NICs (at least for production)
  • V4 dedicated eth1 link is much faster and more robust than V3 serial link
  • V4-HA requires a steep learning curve to fully understand
  • V4 HA installation has been simplified with a helper utility

What does the V4 cluster manage?

  • Apache
  • Asterisk and ALL of its data
  • MySQL and ALL of its data
  • DRBD

None of these components will be available on the passive node (DRBD is running but you can't access its data partition, at least not without a fight). Even if you were to manually start any of these components (don't!), they would not see the live data.

A word of caution

Pacemaker, Corosync and DRBD are extraordinarily powerful and complex pieces of software. You need to understand them and, at least in outline, be familiar with how they work together with one another. The ASHA Utility, makes V4 installation very simple and straightforward and hides a lot of the complexity but you do need to be aware of how to recover from some of the possible failures that can occur when running a cluster. Getting it wrong, or changing parameters without rehearsal can result in data loss so you must have a clear data backup and recovery strategy for your cluster before you make changes.

Installation Prerequisites

  • A minimal Debian Wheezy install (When tasksel runs you should only select openssh and nothing else)
  • An empty partition on each cluster node.
    • The partition MUST be exactly the same size on each node
    • It should be marked as "Do not Use" when you define it.
    • The partition needs to be large enough to hold all of your MySQL and Asterisk data (including room for call recordings if you plan to make them).
    • By convention we define the first logical partition (/dev/sda5) as the empty partition
  • A minimum of 2 NICs on each node

How big should the DRBD partition be?

DRBD partition size is a trade off between making it big enough to handle any eventuality and the time it takes to synchronize when you bring a new or repaired node into the cluster. As a rough guide, on a single dedicated Gigabit link, the cluster will sync at around 30MBs. On our own SARK systems we use a 64Gb SSD drive and we allocate 50Gb to the DRBD partition, 8Gb to root and 4Gb swap. This gives ample room for call recording with periodic offload and it syncs a new node in about half an hour. You can increase the sync speed and resilience by using extra NICs with NIC bonding if you wish but the basic install we discuss here does not provide it.

Installation

Unlike most Pacemaker install guides which describe installing the nodes in tandem the sail install proceeds one node at a time. Thus your first cluster will, somewhat paradoxically, consist of only a single node. This is perfectly fine and it actually makes the creation of the second node considerably easier because Pacemaker and DRBD will do most of the heavy lifting for you automatically.

Once you have your Debian installation, choose a candidate node and install sail V4 using the notes HERE. You may also wish to install your favourite source code editor at this point and any ancillary tools you feel you may need (tshark or whatever).

Now it is time to install the cluster. This is where we introduce ASHA (Asterisk SARK High Availability). ASHA is a little helper utility we built to set up your cluster. 

apt-get install asha

ASHA will drag the openAIS stack as part of its install. It will also ask you a series of questions and use your answers to generate a set of scripts to define your cluster and fetch it on-line.

What does asha need to know?

ASHA will ask the following

  • Whether this is the first node to be defined for this cluster (or a subsequent definition). The generated scripts will be different for the first and subsequent nodes.
  • The virtual IP of your cluster (this will be a free static IP address on the local LAN)
  • Whether you wish to deploy with 1 or 2 NICs. You should NEVER deploy a cluster with a single NIC except for learning and play.
  • The IP address of the cluster communication NIC on this node (usually this will be eth1)
  • The IP address of the cluster communication NIC on the other node
  • The node name (uname -n) of the other node (which you may not have installed yet)
  • An administrator email address for cluster alerts
  • A unique identifier string which will be placed into the email alert subject line

Completing the installation

After the ASHA install completes, if you are feeling brave, you can simply run the installer 

sh /opt/asha/install.sh

For the more prudent, you may wish to examine and understand the scripts before you run them. You will find them in /opt/asha/scripts and the number and contents will vary depending upon whether this is the first node you have defined for your cluster or the second. Here are the scripts for a first node install. 

root@crm1:~# ls /opt/asha/scripts/
10-insert_pacemaker_drbd_sark_rules.sh	50-make_corosync_live.sh
20-bring_up_drbd_first_time.sh		70-pacemaker_crm.sh
30-initial_copy_to_drbd.sh

the scripts are run in order, lets see what they do

10-insert_pacemaker_drbd_sark_rules.sh

This script sets up the cluster communication NIC and sets the shorewall firewall rules to allow it to communicate with its peer.

20-bring_up_drbd_first_time.sh

This will bring up DRBD and (if this is the first node defined), build the DRBD filesystem

30-initial_copy_to_drbd.sh

If this is the first node, the source data (from Asterisk, SARK, MySQL) will be copied into the DRBD partition

50-make_corosync_live.sh

Does what it says. It will start the Corosync cluster communication layer

70-pacemaker_crm.sh

This script will build the Pacemaker CIB (Cluster Information Base) using the Pacemaker CRM (Cluster Resource Manager). The CRM provides an abstracted view of the underlying XML which Pacemaker uses to do its stuff. It is complex, arcane and easily broken. In general you should not touch the CIB until you absolutely know what you are doing and why. You should NEVER touch the underlying XML unless you want to end up crying in the corner of a darkened room. There is lots of CIB documentation available on-line for the curious and the brave.

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