Hot Technologies in Storage

Continued from page: 1

Tuesday, July 03, 2007

Storage Virtualization Consolidation and more

As your business grows, your data also grows exponentially. As a result, the available storage capacity for storing all this data shrinks. So you buy more storage devices to cater to this growing requirement, and the cycle continues. The trouble is that over a period of time, both user preferences as well as the available storage technologies change, and you end up deploying disparate storage resources in your IT infrastructure. Eventually this makes the management of so many storage resources difficult, and often leads to underutilized storage. In fact, it wouldn't be surprising if only 50% of storage capacity is actually utilized. It's a fairly common sight these days. So what do you do with the additional 50% capacity? How do you leverage it?

One answer that the storage industry has for this is storage virtualization, which not only consolidates the storage infrastructure but also makes it more usable. It allows storage administrators to identify, provision and manage disparate storage as a single aggregated resource. The end result is that it eases the management headaches and allows higher levels of storage utilization, which in turn forestalls the expense of added storage. Let's examine this exciting technology in more detail and even look at some of the key issues involved in using it.

Key issues
The technology basically works by adding a layer of abstraction between the storage systems and applications. So, applications no longer need to know what disks, partitions or storage subsystems their data is stored on. They look at the storage as one pool, which results in improved disk utilization. It also helps automate storage capacity expansion, and allows storage resources to be altered and updated on the fly without disrupting application performance.

Earlier, an application would be associated to specific storage resources, and any interruption to those resources will adversely affect the application's availability. After doing storage virtualization, they're no longer tied to particular storage units, thereby improving data availability.

Storage virtualization can also aid in disaster recovery (DR) planning. Traditionally you needed identical hardware at the DR site for replication of data, but virtualization eases that requirement. Moreover, you can speed up backups through the use of snapshots, which basically eliminates the backup window. Data migration can also be handled through storage virtualization instead of using vendor-specific tools, supporting greater heterogeneity in the data center.

But, all this also adds up to the complexity significantly. The virtualization layer is one more element of the storage environment that must be managed and maintained as and when virtualization products are patched and updated. It's also important to consider the impact of storage virtualization on interoperability and compatibility between storage devices. In some cases, the virtualization layer may potentially interfere with certain special features of storage systems, such as remote replication.

If you face such issues with storage virtualization, then undoing it can be very challenging. Therefore it is advisable to go one step at a time. Implement it in parts.

Technologies behind storage virtualization
Currently, storage virtualization is being done at three architectural levels: (a) in the host, (b) at the storage sub-system and (c) in the storage network. Each method provides specific advantages but is limited in its capabilities.

Host based virtualization is the easiest and most straightforward. Abstraction is implemented in servers, typically in Logical Volume Managers (LVM). But scalability and maintainability become an issue in this kind of virtualization, after a while. Reason for the same is that it assumes prior partitioning of the entire SAN resources (disks or LUNs) to various servers.

You can also have storage virtualization in the storage array itself, e.g., Hitachi Data Systems' TagmaStore. This offers convenience, but it's vendor-centric and generally, not heterogeneous. Pooling all SAN storage resources and managing virtual volumes across several storage subsystems generally requires homogeneous SANs using a single type of RAID subsystem.

Today, the most popular point of implementation is in the network fabric itself. It is winning because of its neutrality for storage and servers. It is often done through a dedicated virtualization appliance or an intelligent switch running virtualization software, such as IBM's SVC software. Network-based virtualization can have following two major architectures:

•  Symmetric approach – intelligent switches and/or appliances in the data path of the storage network infrastructure.
• Asymmetric approach - separate appliances installed out of the data path of the storage network infrastructure.The appliance can be a small and inexpensive unit, because it does not have to handle actual data transfers, along with controls.

Network-based storage virtualization is the most scalable and int eroperable; making it particularly well suited to storage consolidation projects. But as a downside, there may be a slight impact on network performance due to in-band processing (symmetric approach) in the virtualization layer.

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