openstack-manuals/doc/arch-design/source/generalpurpose-operational-considerations.rst

Operational considerations

In the planning and design phases of the build out, it is important to include the operation's function. Operational factors affect the design choices for a general purpose cloud, and operations staff are often tasked with the maintenance of cloud environments for larger installations.

Expectations set by the Service Level Agreements (SLAs) directly affect knowing when and where you should implement redundancy and high availability. SLAs are contractual obligations that provide assurances for service availability. They define the levels of availability that drive the technical design, often with penalties for not meeting contractual obligations.

SLA terms that affect design include:

• API availability guarantees implying multiple infrastructure services and highly available load balancers.
• Network uptime guarantees affecting switch design, which might require redundant switching and power.
• Factor in networking security policy requirements in to your deployments.

Support and maintainability

To be able to support and maintain an installation, OpenStack cloud management requires operations staff to understand and comprehend design architecture content. The operations and engineering staff skill level, and level of separation, are dependent on size and purpose of the installation. Large cloud service providers, or telecom providers, are more likely to be managed by specially trained, dedicated operations organizations. Smaller implementations are more likely to rely on support staff that need to take on combined engineering, design and operations functions.

Maintaining OpenStack installations requires a variety of technical skills. You may want to consider using a third-party management company with special expertise in managing OpenStack deployment.

Monitoring

OpenStack clouds require appropriate monitoring platforms to ensure errors are caught and managed appropriately. Specific meters that are critically important to monitor include:

• Image disk utilization
• Response time to the Compute API <Compute API (Nova API)>

Leveraging existing monitoring systems is an effective check to ensure OpenStack environments can be monitored.

Downtime

To effectively run cloud installations, initial downtime planning includes creating processes and architectures that support the following:

• Planned (maintenance)
• Unplanned (system faults)

Resiliency of overall system and individual components are going to be dictated by the requirements of the SLA, meaning designing for high availability (HA) can have cost ramifications.

Capacity planning

Capacity constraints for a general purpose cloud environment include:

• Compute limits
• Storage limits

A relationship exists between the size of the compute environment and the supporting OpenStack infrastructure controller nodes requiring support.

Increasing the size of the supporting compute environment increases the network traffic and messages, adding load to the controller or networking nodes. Effective monitoring of the environment will help with capacity decisions on scaling.

Compute nodes automatically attach to OpenStack clouds, resulting in a horizontally scaling process when adding extra compute capacity to an OpenStack cloud. Additional processes are required to place nodes into appropriate availability zones and host aggregates. When adding additional compute nodes to environments, ensure identical or functional compatible CPUs are used, otherwise live migration features will break. It is necessary to add rack capacity or network switches as scaling out compute hosts directly affects network and datacenter resources.

Assessing the average workloads and increasing the number of instances that can run within the compute environment by adjusting the overcommit ratio is another option. It is important to remember that changing the CPU overcommit ratio can have a detrimental effect and cause a potential increase in a noisy neighbor. The additional risk of increasing the overcommit ratio is more instances failing when a compute host fails.

Compute host components can also be upgraded to account for increases in demand; this is known as vertical scaling. Upgrading CPUs with more cores, or increasing the overall server memory, can add extra needed capacity depending on whether the running applications are more CPU intensive or memory intensive.

Insufficient disk capacity could also have a negative effect on overall performance including CPU and memory usage. Depending on the back-end architecture of the OpenStack Block Storage layer, capacity includes adding disk shelves to enterprise storage systems or installing additional block storage nodes. Upgrading directly attached storage installed in compute hosts, and adding capacity to the shared storage for additional ephemeral storage to instances, may be necessary.

For a deeper discussion on many of these topics, refer to the OpenStack Operations Guide.