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Storage virtualization--architectural considerations, Part 2 of 3.

In Part One of this series, we discussed how storage virtualization can deliver a non-disruptive operating capability, enabling users to address the increasingly critical challenge of reducing the planned downtime associated with making changes to their storage infrastructure.

By way of review, storage virtualization provides a physical-to-logical storage device abstraction. It presents a simple, consistent representation of a complex infrastructure to the entities consuming resources. Virtualization is an inherent capability of enterprise storage arrays, which aggregate capacity from multiple fixed disk drives in a single physical frame and present logical volumes for host access. More recently, a new class of virtualization technology has emerged that virtualizes capacity from multiple heterogeneous arrays across the entire SAN and manages a logical representation of this capacity from a single point.

However, there are many different approaches and challenges associated with this new class of virtualization. Architecture plays a defining role as to the ultimate value of these solutions in enterprise environments.

Host-based Storage Virtualization

One solution for addressing the challenge of aggregating and managing capacity from multiple SAN-based devices is one already deployed in many end-user environments: the host-based logical volume manager (LVM). Indeed, LVMs are becoming a standard part of most modern server operating systems. LVMs are software utilities that manage logical volumes presented from various storage devices, configuring capacity to suit the needs of an application. For example, these may concatenate a set of volumes configured at a small size at the array level to present a single large volume, they may slice a large array volume into several more manageable units, or they may be used to stripe data across a number of array volumes for performance reasons, while maintaining a single representation of the capacity to the application.

While LVMs provide some of the benefits of larger-scale, multi-device virtualization, they carry with them an intrinsic limitation--they are host-based and, as such, configuration and deployment must be done individually for each host. This is not an issue if there are a small number of hosts, but in an enterprise setting, where there are typically hundreds or even thousands of hosts accessing SAN-based storage, the manageability of this distributed capability quickly becomes challenging. This issue is exacerbated in environments with a large degree of change, which necessitate frequent configuration modifications. Manageability also is a challenge if different LVMs are deployed across different operating systems, requiring administrators to be proficient with multiple toolsets. Other challenges that emerge when using host-based approaches are interoperability (making sure that third-party LVMs stay compatible with operating system revisions and new devices) and performance (some intensive LVM operations can sap host processing cycles).

Network-based Storage Virtualization

Network-based virtualization architectures attempt to address some of the challenges inherent in the host-based model. By putting the virtualization functionality in a layer between the hosts and subsystems, the functionality is more centralized for easier manageability. There are two architectural approaches: in-band and out-of-band.

In-Band Approaches

In-band architectures insert a virtualization device in the network data path (or "in-band") between the hosts and the arrays. These devices typically offer volume management and other complementary functionality, such as data movement and copy services. In effect, they act as replacement storage controllers for the devices they are virtualizing. The virtualization device itself can be a dedicated server running virtualization software installed on top of a standard operating system, a dedicated appliance running embedded code, or even an array controller "front-end" with a back-end that permits connection of additional array frames. The primary advantage of this approach is simplicity--one (new) self-contained device can be deployed to act as a central point of management for multiple connected devices.

One basic disadvantage of the in-band approach is the addition of an extra "hop" to the network path, which adds latency between the hosts and the physical storage. Some in-band devices attempt to address the added latency by employing caching within the device itself. Caching within the network also carries with it additional complications. For high-availability environments that demand redundancy, preserving cache coherency between a pair of in-band devices requires cache mirroring, which adds back some latencies. It also requires robust error and failure handling logic to ensure that cached and acknowledged I/O is stored safely on the back-end device.

A more significant disadvantage of in-band virtualization architectures is a limit to scalability. Since all I/O within the virtualization domain needs to go through the in-band device, it can become a bottle-neck, either in terms of bandwidth or processing power. Once either resource becomes depleted, a scaling strategy must be employed. As we noted above, the need to mirror cache across in-band nodes makes "scale out" strategies (in which n-number of additional nodes are added for scaling) impractical. Instead, the only practical recourse is a "scale up" strategy that calls for bigger and bigger in-band nodes to deliver in-band virtualization on a large scale. At some point for a large environment, even a "scale-up" strategy will prove to be insufficient and a new in-band device will need to be deployed.


Out-of-band approaches are designed to avoid the performance challenges inherent in an in-band architecture by separating the management information from the data flow itself. In an out-of-band architecture, a separate piece of hardware called the metadata server, which contains information about the logical-to-physical relationships of the virtualized storage, communicates to each server, informing it where to direct its I/O requests. This communication is done over an independent network, separate from the Fibre Channel network used by the data traffic--hence the out-of-band description.

Because the host issues requests for virtualized storage directly to the destination device, the I/O performance is free of additional latency or bandwidth bottlenecks. Thus, the out-of-band approach is theoretically more suitable for higher performance applications. It also avoids the data integrity issues inherent with the in-band approach. No "state" or version of the data is ever held in the network. Until the data is properly stored on the array, the host is not made to believe the job has been completed. However, this type of out-of-band approach reintroduces some of the manageability challenges of the host-based approach. Namely, the need to load, maintain and qualify host-based software.

A refined out-of-band approach is emerging that addresses this manageability challenge. This approach leverages intelligent SAN switches as the platform for deployment of network-based storage virtualization. These switches have specialized port-level processors (frequently optimized ASICs, but also could be FPGAs or network processors) that inspect and redirect I/O (translate from logical to physical addresses) at wire-speed. By incorporating these processors directly into the existing SAN fabric, the need to manage another layer of virtualization devices is obviated. The metadata which was formerly managed at the host by the host agent is loaded into flash memory at the intelligent port, obviating the need for host-based software. Instead of communicating with the hosts, the metadata server communicates with the intelligent ports, ensuring they always have the right mapping information for the hosts accessing storage through those ports. In sum, the manageability of this refinement is greatly increased.

This switch-based out-of-band approach is also much more amenable to a "scale-out" scaling strategy. Because the vast majority of I/O processing (and, thus, the capacity of the implementation) is handled directly by the port-level processor in the intelligent switch, when increased scale is required, all that needs to be done is add more processors. This can be accomplished by adding another switch to the fabric or by adding another processing blade to an existing switch. The additional processors are still managed by the same metadata server, which does not need to scale nearly as often, as it does not handle I/O traffic, but rather only manages the metadata across the ports. In short, this architecture is theoretically capable to scaling to very large configurations, the kind of scale that will be needed to extend the benefits of storage virtualization across today's largest data centers.


There are a number of approaches to storage virtualization, each with their own attributes. As we have shown, architecture can be a key determinant of a storage virtualization solution's manageability, scale and ultimately, value to its adopter. A full understanding of a solution's architecture should be a key consideration for any potential adopter of this technology.

Mark Lewis is executive vice president and chief development officer at EMC Corporation (Hopkinton, MA).

Please note: This is the second article in Lewis' three-part series on virtualization. We will return to this question and pose several more that should be asked of any potential virtualization vendor in the next and final article in this series.
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Title Annotation:Storage Management
Author:Lewis, Mark
Publication:Computer Technology Review
Date:Jun 1, 2005
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