By Stephen J. Bigelow, Senior Technology Writer
This article is part of the Virtualization Explained series. For more information, check out our related articles on server virtualization and application virtualization.
Inefficient storage utilization and unnecessary storage purchases have led to the adoption of storage virtualization solutions, which use abstraction to separate physical disk space from the logical assignment of that space. Virtualization allows the storage available on multiple systems (often SAN storage subsystems) to be aggregated so that disk space forms a single logical resource. The virtualized storage pool is then provisioned for use by users, servers and applications.
The basic premise of storage virtualization solutions is not new. Disk storage has long relied on partitioning to organize physical disk tracks and sectors into clusters, and then abstract clusters into logical drive partitions (e.g., the C: drive). This allows the operating system to read and write data to the local disks without regard to the physical location of individual bytes on the disk platters.
Virtualization brings efficiency to the storage environment. By pooling storage resources into a single resource, administrators can manage all of the space included in the pool regardless of its location. This allows for much better storage utilization, often reaching 80% or better. Storage management is also easier since virtualized storage can be managed from a single administrative console -- a storage administrator can see utilization trends and growth patterns more clearly and make better upgrade or capacity planning decisions. Virtualized storage can be thinly provisioned, or even dynamically provisioned (increasing or decreasing the size of a LUN) as needed.
Virtualized storage is also flexible. Once storage space is decoupled from a physical disk or storage array, it's simple to migrate and copy that virtual storage between systems or geographic locations. For example, a virtual LUN can be migrated from an older storage system to a newer one for better performance without making any adjustments to the application. Similarly, a virtual LUN can be copied to another local storage system for backup purposes, or replicated to an off-site storage location for disaster recovery purposes.
There are numerous elements involved in storage virtualization solutions. They can be implemented within the storage array itself using products like the Universal Storage Platform V from Hitachi Data Systems (HDS) or the EqualLogic PS family from Dell, or through dedicated appliances like the Virtual SAN Appliance (VSA) for VMware from LeftHand Networks. There are also software-based storage virtualization products, such as the SANsymphony software from DataCore Software, Invista software from EMC Corp. or the Veritas Virtual Infrastructure from Symantec Corp.. There are numerous other products available to implement storage virtualization -- each with its own unique features and pros and cons. Solution providers face the challenge of matching each customer's needs with the most suitable virtualization product.
What are the issues or limitations with storage virtualization?
Entry costs and learning curves are the first issues for smaller businesses. Client costs usually start with a network assessment and deployment of any appropriate network upgrades for the LAN (e.g., moving some elements of the LAN from 1 Gigabit Ethernet to 10 GigE) or SAN. Higher storage utilization often means higher storage traffic from users and higher migration traffic between storage systems, so ensuring network readiness is always the first consideration for a solution provider.
Solution providers must also consider the interoperability of virtualization products within the customer's storage environment. "If you have heterogeneous storage vendors in the mix, there's an ambition for everyone to support everyone else's equipment," said Keith Norbie, director of storage and virtualization at Nexus Information Systems in Plymouth, Minn. "The question is, how well does that work?"
While there can be significant benefits to virtualizing a heterogeneous storage environment, Norbie cautions that it's not always possible. Often, solution providers face performance and support issues during a heterogeneous storage virtualization project. It may be necessary to avoid virtualization on some storage systems until a future technology refresh cycle allows the client to implement a more virtualization-friendly storage platform.
Once assessments and upgrades have been completed, costs then move to the deployment, configuration and testing of storage virtualization products. Training should also be provided so that customers can work within the virtualized environment. Losing the relationship between data and physical storage can be disorienting, and customers need to understand how to use new storage management tools. Many solution providers ease their customers' entry into virtualization by deploying the technology in phases -- starting with a single storage system or application, refining the deployment and systematically extending the virtualization to other storage over time.
