Scale-out vs. scale-up: the basics

Randy Kerns — Wed, 23 Feb 2011 19:08:41 +0000

There’s been a lot of talk about scale-out and scale-up storage lately, and I get a sense that a lot of people don’t understand that these terms are not synonymous. And that causes confusion among IT professionals when they are planning product purchases and trying to determine how these types of products bring value versus cost and complexity to their environments.

To make informed buying decisions, IT pros need to understand the difference between scale-up and scale-out. The following are the basics, which can be built upon for more detailed considerations.

Scale-up, as the following simple diagram shows, is taking an existing storage system and adding capacity to meet increased capacity demands.

Scale-up can solve a capacity problem without adding infrastructure elements such as network connectivity. However, it does require additional space, power, and cooling. Scaling up does not add controller capabilities to handle additional host activities. That means it doesn’t add costs for extra control functions either.

So the costs have not scaled at the same rate for the initial storage system plus storage devices – only additional devices have been added.

Scale-out storage usually requires additional storage (called nodes) to add capacity and performance. Or in the case of monolithic storage systems, it scales by adding more functional elements (usually controller cards).One difference between scaling out and just putting more storage systems on the floor is that scale-out storage continues to be represented as a single system.

There are several methods for accomplishing scale out, including clustered storage systems and grid storage. The definitions of these two types can also be confusing, and other factors add to the complexity (that’s a subject for another article), but the fundamental premise is that a scale-out solution is accessed as a single system.

This diagram shows an example of a scale-out storage solution. In this diagram, the scaling is only with an additional node but a scale-out solution could have many nodes that are interconnected across geographical distances.

The scale-out storage in this example added both the control function and capacity but maintained a single system representation for access. This scaling may have required additional infrastructure such as storage switches to connect the storage to the controller and a connection between the nodes in the cluster or grid. These connections let the solution work as a single system.

Scaling-out adds power, cooling, and space requirements, and the cost includes the additional capacity, control elements and infrastructure. With the scale-out solution in this example, capacity increased and performance scaled with the additional control capabilities.

Not all scaling solutions are so simple. Many storage systems can scale out and up. The following diagram illustrates this:

Considerations

When looking at scale-up or scale-out storage, consider these factors:

Costs Scale up adds capacity but not the controller or infrastructure costs. If the measure is dollar per GB, scale-up will be less expensive.
Capacity. Either solution can meet capacity requirements but there may be a limit on the scale-up capacity based on how much capacity or how many devices an individual storage controller can attach.
Performance. Scale out has the potential capability to aggregate IOPS and bandwidth of multiple storage controllers. Representing the nodes as a single system may introduce latency, but this is implementation specific.
Management. Scale up would have a single storage system management characterization. Scale-out systems typically have an aggregated management capability but there may be variations between vendor offerings.
Complexity. Scale-up storage is expected to be simple, while scale-out systems may be more complex because they require elements to manage.
Availability. Additional nodes should provide greater availability in case one element fails or goes out of service. This depends on the particular implementation.

There is a great deal to consider when making a choice between scale out and scale up. The decision will ultimately depend on how one vendor implements its solution and its capabilities and features compared to another vendor. But, it is always best to start with a basic understanding and then look at the differences.

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Scale-out vs. scale-up: the basics