In the Linux world, a popular approach to build highly available clusters is with a set of software tools that include pacemaker (as resource manager) and corosync (as the group communication system), plus other libraries on which they depend and some configuration utilities.
On Illumos (and in our particular case, OmniOS), the ihac project is abandoned and I couldn’t find any other platform-specific open source and mature framework for clustering. Porting pacemaker to OmniOS is an option and this post is about our experience with this task.
The objective of the post is to describe how to get an active/passive pacemaker cluster running on OmniOS and to test it with a Dummy resource agent. The use case (or test case) is not relevant, but what should be achieved in a correctly configured cluster is that, if the node of the cluster running the Dummy resource (active node) fails, then that resource should fail-over and be started on the other node (high availability).
Fig. 1: Redundant MySQL Server nodes using Pacemaker, Corosync and DRBD.
If automation is required, Vagrant and Puppet seem to be the most adequate tools to implement it. What about automatic installation of High Availability database servers? As part of our Cloud Dependability efforts, the ICCLab works on automatic installation of High Availability systems. One such HA system is a MySQL Server – combined with DRBD, Corosync and Pacemaker.
In this system the server-logic of the MySQL Server runs locally on different virtual machine nodes, while all database files are stored on a clustered DRBD-device which is distributed on all the nodes. The DRBD resource is used by Corosync which acts as resource layer for Pacemaker. If one of the nodes fails, Pacemaker automagically restarts the MySQL server on another node and synchronizes the data on the DRBD device. This combined DRBD and Pacemaker approach is best practice in the IT industry.
At ICCLab we have developed an automatic installation script which creates 2 virtual machines and configures MySQL, DRBD, Corosync and Pacemaker on both machines. The automated installation script can be downloaded from Github.
As proposed in a former article different technologies must be evaluated in order to make the current MobileCloud environment suitable to High Availability (HA) requirements. The following article lists a basic evaluation of the different technologies that could be used.
Basically there are four technologies which allow to build a reliable HA-infrastructure for OpenStack:
Build OpenStack on top of Corosync and use Pacemaker cluster resource manager to replicate cluster OpenStack services over multiple redundant nodes.
For clustering of storage a DRBD block storage solution can be used. DRBD is a software that replicates block storage (hard disks etc.) over multiple nodes.
Object storage services can be clustered via Ceph. Ceph is a clustered storage solution which is able to cluster not only block devices but also data objects and filesystems. Obviously Swift ObjectStore could be made highly available by using Ceph.
OpenStack has MySQL as an underlying database system which is used to manage the different OpenStack Services. Instead of using a MySQL standalone database server one could use a MySQL Galera clustered database servers to make MySQL highly available too.
The different technologies have been evaluated according to their ability to make different OpenStack components highly available. The following table shows which technologies could be used to make the different OpenStack Services used in MobileCloud suitable to High Availability requirements.
Table 1.1: OpenStack Services and Clustering Technologies which make them suitable to HA requirements.
It is obvious that the different technologies can be used in different architectural setups. It is obvious that they must be used in a multi-node OpenStack Architecture. An architecture proposal will follow up in a further article.
Open Stack’s capabilities to support High Availability are very limited. If a virtual machine crashes, there is no automatic recovery. Clustering software seems a to be a great workaround to allow redundancy and implement High Availability (HA).
Pacemaker is a scalable cluster resource manager developed by Clusterlabs. Its advantages are:
Support of many different deployment scenarios
Monitoring of resources
Recovery from outtages
According to the OpenStack documentation website the OpenStack HA environment builds on Pacemaker and Corosync. Corosync is Pacemaker’s message layer which is responsible for the distribution of clustering messages. The Pacemaker software uses resource agents that manage different ressources and communicate via Corosync. Corosync is responsible for synchronizing DRBD block devices which are virtual devices layered on top of the machine node devices themselves (like hard-disks etc.). The DRBD block device layer allows clustering of different machine nodes, while Corosync organizes the synchronicity of data in these clusters. Pacemaker resource agents control the DRBD devices via Corosync and are therefore able to organize high availability of machine nodes in an OpenStack environment.
Integration of Pacemaker into OpenStack is a major step towards creating a HA cloud environment. There’s an ongoing evaluation how Pacemaker fits into the MobileCloud environment, but it is obvious that there should be a test procedure to evaluate availability of cloud resources in different integration scenarios. Follow up information on this subject will be posted in a further blog post.