Storage Pooling In Storage Area Networks.

No doubt many of you are dealing with pre-SAN configurations and are confronting the cabling and bandwidth constraints of SCSI hardware. The most frustrating of these limits include short segment lengths (30 meters) and the inability to address more than a few devices per host bus adapter. Such hardware limitations inherently make disks slaves to individual servers and create wiring nightmares. In Figure 1 for example, the middle server has exhausted its disk space, while adjacent servers have much surplus. Yet because these resources are bound to specific hosts, we have no way to reallocate their spare capacity.

Fibre Channel Alone Is Not

Enough With Fibre Channel products came the introduction of Storage Area Networks (SANs), and cabling became much easier.

As illustrated in Figure 2, fewer cables stretching dramatically longer distances attaching numerous devices were a substantial improvement in connectivity. Fibre Channel switches also addressed fault isolation and device segmentation in an intelligent way.

Nevertheless, the disks remain enslaved to individual servers, with no relief for the applications that are running out of capacity.

On the face of it, one would think that the connectivity afforded by Fibre Channel would have allowed disks to be freely shared. However, there are several technical issues having to do with the underlying control of inherently SCSI devices:

* Hosts communicate with physical storage devices using SCSI protocol over Fibre Channel. The destination address is a combination of a Target I.D. and a Logical Unit Number (LUN).

* Some hosts take ownership of any visible LUNs on attached cables (fibers).

* Two or more hosts connected to the same LUN will unknowingly compete for the same storage device.

To deal with these issues, you'll find that Fibre Channel (FC) switches are often zoned to block other servers from connecting to the same storage device. Another workaround, known as "LUN Masking" puts the responsibility for ignoring neighboring servers' LUNs on each application server. This scheme relies on special host-based device drivers to provide a filtered view of the storage network. Those hosts who are either unable or unwilling to have the additional LUN Masking software installed remain a threat to the security and integrity of the stored data. Of course, this is unacceptable. Hosts' inability to share storage assets thus prevents them from taking advantage of each other's surplus capacity and ripples into application downtime and cumbersome backup chores.

Virtualization Is The Answer

SAN virtualization is the additional ingredient that enables network storage pooling. Physical disks are first virtualized into logical volumes to abstract their properties and ownership. Multiple logical volumes may be carved out of each physical disk, arbitrarily sized to meet application needs. Alternatively, two or smaller physical devices can be aggregated into a larger logical disk. These logical volumes now represent discretely addressable capacity that may be freely allocated to servers on demand.

For example, DataCore's SANsymphony software makes the allocation of logical volumes to specific servers a simple and intuitive activity using the drag-and-drop GUI. A central storage administrator controls the capacity management from the secure GUI on behalf of all requesting application servers. A simple click and drag of the mouse in the SANsymphony virtual mapping screen thus replaces the painful process of scheduling a maintenance window, shutting down a server, connecting additional disks, and restarting the hosts.

The simpler configuration in Figure 3 is intended to illustrate the role of each element. More sophisticated networks would be configured to ensure highly available access to the storage resources. Note that the storage devices have been zoned to the storage domain server, as have the application servers. This is how the servers are blocked from seeing the actual physical disks. They can only see the logical volumes (not shown) allocated to them by the storage domain servers. To the application servers, the logical volumes appear as well behaved directly attached disks. These virtual disks can be formatted and file systems or databases mounted on them just like any conventional physical disk.

Powerful Virtual Attributes

The virtual disks do not merely redirect the I/O stream; they also enhance the attributes of the physical devices from which they originate. Generally, their response time is faster through caching services inherent in the storage domain servers. So too is their availability. The virtual disks can optionally include host and storage device-independent snapshot images and remotely mirrored copies to simplify data sharing with backup/recovery utilities and other data consumers. All these value-added properties are available to the storage administrator on a per volume basis.

Through SAN virtualization, heterogeneous network storage pools leverage Fibre Channel's connectivity, extended distance, and high-bandwidth traits to provide functionality and benefits not previously possible. The virtual network disks incorporate the necessary characteristics to enable:

Vastly improved capacity utilization through storage resource sharing, which means lower costs.

* Drag-and-drop capacity allocation, which means easier storage management and lower operational costs.

* Responsiveness to rapid storage expansion and change, which means better fulfillment of business needs.

* Network software-based capacity management, which means no more application downtime for risky and complicated hardware configuration.

Angie Gonzalez is the director of product marketing at Data Core Software (Fort Lauderdale, FL).