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Issue of July 2002 
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Vendor Voice: Storage Virtualization
Virtualization in 'non-SAN' environments

Although virtualization is often associated with SANs its benefits are not new and can also be realized in 'non-SAN' environments. by Savio Monteiro

Storage virtualization is yet another example of the old becoming new. Although the concept is finding new life and importance with the emergence of SANs, storage virtualization technologies are widely used in both mainframe and open systems environments to help simplify administration and provide flexibility in demanding storage environments. Amidst all the excitement about SAN virtualization, it's easy to lose sight of the benefits of storage virtualization in the prevailing DAS (Direct Attached Storage) architecture.

Despite the hype and new product introductions, SANs have not yet been widely adopted in corporate IT. The Enterprise Storage Group estimates that in large U.S. companies, only about 5 percent of storage is in a Fiber Channel SAN. The remaining 95 percent is DAS. If you're handling that 95 percent today, storage virtualization can offer immediate relief in terms of storage manageability and flexibility.

Virtualization combines multiple physical storage devices into logical (virtual) storage devices or units

Defining virtualization
Despite the recent surge in interest, storage virtualization is not new, either in concept or in practice. Defined almost 20 years ago in a technical white paper created by an IBM mainframe users' group, the concept of storage virtualization was put into practice in the MVS operating system so that strings of DAS could be addressed as simpler, logical entities.

Storage virtualization is the process of taking multiple physical storage devices and combining them into logical (virtual) storage devices or units that are presented to the OS, applications, and users. In a sense, storage virtualization builds a layer of abstraction above the physical storage (see diagram).

The important part
The important part is what can happen in this abstraction process. Since data is not tied to specific hardware devices, virtualization provides a very flexible storage environment. It simplifies the management of storage and can potentially reduce costs through better hardware utilization and consolidation.

The virtual devices are not restricted by the capacity, speed, and reliability limitations of the physical devices. By applying intelligent storage software in the virtualization layer, virtualization offers a way to address the functional challenges of storage.

Users are not generally interested in the physical aspects of the storage serving their applications. They don't want to hear about seek times or rotational latency. They don't care how many disks are in a string or the mean time between failure for those disks.

What they do care about are issues of application response time and throughput, sufficient capacity for their data as it grows, and application downtime. In short, they care about the application aspects of their data, not the physical aspects of storage.

The virtualization layer offers a chance to combine physical devices into virtual entities that meet application requirements. For example, you can create a device that optimizes performance for a specific application, while shielding users and applications from the physical details of the implementation. As new hardware becomes available or application characteristics change, you can modify the physical layer without interrupting access to the logical device.

One of the benefits of storage virtualization, particularly in a SAN environment is that it enables consolidation, which simplifies management and possibly reduces the total amount of storage managed.

Some organizations create SLA (Service Level Agreements) with their constituents which promise specific levels of availability and performance. Administrators, often with their jobs on the line, have to operate conservatively. They would rather over-provision than under-provision storage for a critical application. This leads to an underutilization of storage. According to Forrester Research, many organizations use only about 50 percent of their disk space.

At the same time, they may buy proprietary storage systems that promise certain levels of availability or performance and then find themselves with additional management tasks for those proprietary storage systems. All of this can lead to a complex administrative environment.

Logical volume manager
In an open systems environment, logical volume managers virtualize storage by consolidating physical storage into logical volumes, which are available to applications or file systems. Logical volume managers are available on most Unix platforms (including Linux) and on Windows 2000.

By assembling virtual storage volumes out of numerous physical devices, you can create storage configurations tuned for specific applications, without the limitations of specific hardware devices. Instead, you can make use of whatever storage is at hand, without locking into proprietary storage solutions.

Logical volume managers improve application availability by building redundancy into the logical volume itself. The possibilities go beyond simply mirroring and RAID. For example, the failure of a device in a redundant storage configuration can degrade performance and expose the data to risk from another failure. The logical volume manager can maintain a pool of spare disks that can be automatically swapped in (hot relocation) when a device fails in a logical volume. It can even move data back automatically when the failed device is replaced or repaired.

Unlike a physical device, a logical volume has a nearly limitless capacity: Administrators can add storage as needed without interrupting access to the data. When used with a database's auto-extend capabilities or a file system's automatic extension, a logical volume manager can significantly ease the problem of provisioning storage for growing applications.

Beyond the single system
The potential benefits of storage virtualization can easily extend beyond any single system's needs. Since individual administrators typically manage multiple systems, administration can become a bottleneck in terms of storage scalability.

The cost of managing storage is non-trivial, and over time, exceeds the purchase price of the storage hardware. One way to monitor this cost is to evaluate the amount of storage managed per administrator. Efficient organizations may have one administrator per terabyte of storage or better, while others with more difficult environments may have much lower numbers. Storage virtualization can simplify the management of storage in heterogeneous environments, ultimately reducing the true cost of the storage.

Storage virtualization through logical volume management is simply software. Although it needs to cooperate closely with the host OS there is no reason why it should be tied to any specific OS platform or storage device. A logical volume manager should have cross-platform capabilities and be hardware-agnostic, so organizations can support heterogeneous environments and mix-n-match storage as needed.

Cross-platform advantages
A cross-platform storage virtualization solution offers distinct advantages for administrators or organizations handling multiple OSs like a single interface for managing storage across platforms and devices, centralized management of distributed storage resources, and online administration.

Armed with these tools, organizations can create consistent business methodologies for managing application storage, regardless of platform. For example, companies using virtualization for database storage can define corporate-level policies for the logical volumes and file systems that contain database data and components, mandating certain levels of redundancy and performance.

At the same time, the separation of the logical and physical provides flexibility to change or reconfigure storage hardware transparently, perhaps swapping in higher-performing devices or moving storage to the systems that most need it, without interrupting access to the data.

So, storage virtualization, even in the DAS, open systems environment, helps IT organizations achieve the SLAs they commit to, while simplifying system management and support growing heterogeneous systems. And this kind of virtualization can be extended to shared-storage environments like:

  • Availability clusters that share a connection to the same logical volumes, but 'roll over' ownership when a primary server fails.
  • Shared data clusters in which multiple servers share access to data residing on a single virtual volume. Web servers accessing a common copy of a site is a one good example of this kind of cluster. Clusters can share data either through simple switched SCSI devices (for smaller clusters) or through a SAN.
  • NAS devices hosting file systems that may be shared among multiple servers.
  • SANs.

Savio Monteiro is Country Sales Manager, Veritas Software

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