Market Overview
The demands on and for storage are increasing and, commensurately, the challenge to effectively control the way vast amounts of data are created, stored, and accessed is intensifying. To address data growth without interrupting business operations, rapid deployment of storage and IT resources to meet increasing demand becomes a function of scalability. The primary objective of any type of scaling, therefore, is to create a dynamic, non-disruptive environment that supports data growth in such a way that system capabilities remain as balanced and productive as possible.
Scale-out Introduced
As computer and storage prices drop and performance and capacities continue to increase, low cost “commodity” systems are being used for high performance and high capacity workloads that previously could only be handled by scaling up to much faster storage devices or even to high-performance computers. Scale-out storage allows many small, low cost computers and commodity storage components to be combined and configured to either create an aggregate storage pool or to increase computing power and exceed that of a single traditional storage array or high-performance computer. This commodity-based scale-out model is offered by most vendors and is becoming popular, increasing demand for storage virtualization, shared data storage, and improved data protection services. Clusters and grids are examples of scale-out systems, but “scale-out” can apply just as well to SANs as it does to the NAS environments with which it is more commonly associated.
Scale-out Analysis
Scaling-out has the potential to change the face of data center operations by displacing large, costly enterprise storage equipment with a pool of less expensive devices. It differs from traditional architectures in that it adds new computing, networking, or storage resources to an environment; this is in contrast to scaling up, which can either consolidate several smaller devices into one larger device or simply expand that one device.
Scaling out is accomplished by adding additional nodes to a system—such as a server, a network switch, or a storage device. When you upgrade your PC to a faster model with higher capacity disk drives, you are scaling-up, not out. Scaling out would be expanding a two server system to four servers in order to increase overall compute power. Adding more storage devices to create a larger storage pool is therefore an example of scale-out storage just as creating more network ports by adding more switches is an example scale-out networking.
Essentially, scale-out solutions let users create larger shared pools of compute power and storage capacity to grow well beyond the limits of an individual node. However, they also have the potential to simultaneously create a more complex management environment simply as a result of all the interconnected pieces; generally, therefore, scale-out storage is usually packaged with a corresponding software management and virtualization/abstraction layer that can make hundreds or thousands of nodes behave like a single system (using a global namespace) and that allows the storage resources to be deployed as needed.
Scaling Storage Topologies
Although scale-out is often assumed to be synonymous with NAS and scale-up is, in turn, often assumed to be synonymous with SANs, the truth is that scalability is not a direct function of storage type; scale-out can therefore apply to all three major storage topologies:
DAS: As the first widely popular storage model, DAS (direct attached storage) implementations still comprise a large proportion of the installed storage capacity on all major operating systems. DAS is simpler to manage and can scale out—at least to a limited degree—for added performance and capacity by adding more, larger, or faster drives or by cascading additional control units; however, DAS doesn’t provide many of the efficiencies and advanced functionalities that simplify management efforts and add operational advantages as the storage pool grows.
NAS: NAS (network attached storage) is a special purpose architectural approach comprised of servers, disks, and management software dedicated to serving files over a network. This is why scale-out storage is frequently associated with NAS systems that scale to become a cluster of NAS nodes: these nodes allow for capacity and performance to scale either in combination or independently as needed and yet—this is a crucial point—still maintain a single system image. Unstructured data, normally in file format, is already a huge proportion—probably the majority—of all digital storage and is expected to significantly outpace the growth of structured data and hence increase demand for low-cost scale-out storage. Much of this data resides on NAS.
SAN: Scale-out storage is also an important aspect of SANs (storage area networks) and is usually achieved by adding more SAN switches and creating a scale-out SAN fabric. Users can still achieve linear scale by increasing performance “horsepower” and capacity in step. SANs provide excellent scale-out capability for large enterprises that anticipate significant growth in information storage requirements and, unlike direct attached storage, any excess capacity in SANs can be pooled to provide higher utilization of resources as well as advanced functionalities. SAN topologies can be implemented to meet different needs for SAN scaling: they can scale-out to connect SAN islands built around individual switches into larger single fabrics or can use routers to physically connect the switches while logically isolating the fabrics. Virtualization is also used heavily with SANs.
Making a Choice
The key for any organization searching for an optimum solution is to understand which of its goals are most important. As with so much in IT, the answer is “it depends.” Table 1 is a quick summary of the factors a user might consider in terms of the available topologies and their associations with the different scaling approaches. The eventual decision will almost certainly take other factors into account; these might include functionality, price, TCO, and skill/comfort levels as well as predicted changes and growth across the organization. Scale-out and -up vendors will battle over their relative values in terms of overall economic efficiency and “green-ness.”
What’s best will ultimately depend on the nature of the application: both scaling approaches are valuable tools for IT in general and storage in particular, and many organizations will continue to employ both (separately or in combination) for the foreseeable future as they offer useful, though varying, attributes. For instance, scaling up an existing system often results in simpler storage management than with a scale-out approach as the complexity of the underlying environment is reduced or at least known. That said, scale-up storage systems can only scale as far as the performance and capacity limits of individual storage resources permit.
The Bigger Truth
Putting things simply, to support growth, a user can either add infrastructure capabilities to the current devices (scale-up) or add more devices to the environment (scale-out). Scale-out storage can improve IT management’s ability to provide timely provisioning, greater resource utilization, required performance, and higher levels of data and system availability non-disruptively. The price differential between the two models is currently favoring scale-out computing for those applications that can embrace it, thanks largely to its use of commodity components. In many environments, it is worth considering the two approaches in combination; what is certain is that scaling up and out will continue to play ever larger roles in IT as the demands to service business growth in a flexible manner climb ever higher.
