Heterogeneous SANs: the "Circe" of storage

Computer Technology Review, Feb, 2004 by Stephen Terlizzi

The ability to choose best-of-breed solutions. Lower acquisition costs due to price competition by avoiding vendor lock-in. True high capacity utilization (80 %) through single-step provisioning and fine-grain virtualization. Decreases in storage costs by almost 25% through tiered-storage architectures that put the right data on the most appropriate storage. The seductive call of heterogeneous storage area networks (SANs).

For the IT organizations that fall entranced by this call, many often find that the reality of heterogeneous SANs is not so beautiful--making heterogeneous SANs the Circe of the storage industry. Circe was a legendary enchantress in Greek mythology whose charms few could resist. She enticed passing sailors to her abode only to transform them into beasts. Like Circe, heterogeneous SANs often change storage administrators into beasts of burden.

Problems with Heterogeneous SANs

Direct-attached storage (DAS) couldn't be simpler. Attach a disk array to a server, configure the LUNs, create the file system, and you are on your way. The SCSI protocol is elegant in its simplicity. That's because SCSI was designed just for this environment and not for a switched environment, such as a storage area network.

SCSI's introduction into a SAN environment creates a level of complexity because it places a point-to-point protocol designed for dedicated SCSI cables into a shared transport environment. Even for homogeneous SAN environments, this requires additional SAN security through LUN masking or zoning. As seen in Figure 1. storage administrators need to carefully map out the connections between the servers (and their respective host bus adapters, or HBAs) and the storage. Improper SAN security can lead to data loss or rogue servers gaining access to confidential data.

This problem escalates geometrically (n X m, where n is the number of servers and m is the number of storage devices) as you add more servers and storage to a single SAN. And this equation only takes into account the number of physical devices, when it's really the number of paths between HBAs in the hosts and volumes available from the storage that drive complexity. Moreover, there may be multiple paths to the same volumes to the same host or to different hosts in the case of clustered environments. Layering read-write access rights onto each path, along with the various implementations of dynamic multi-pathing software and high availability implementations within storage controllers, further complicate the environment.

Now introduce heterogeneous storage elements into the environment, different hosts with different operating systems and HBAs, different Fibre Channel switches and different storage devices. Suddenly, a complex but conceivably manageable environment becomes a management nightmare. The beast of burden transformation begins in earnest.

[FIGURE 1 OMITTED]

First, the easier part. Each of the storage elements in the infrastructure has its own element manager. While not ideal, the storage administrator will need to learn how to manage the various elements in the infrastructure via each individual element manager. Fortunately, device management complexity only scales linearly, based on the new devices introduced into the network. Moreover, with management frameworks, this can be handled satisfactorily.

Now, the more difficult part. Interoperability becomes nearly impossible to achieve. As new devices are added to the network, they add complexity into the interoperability matrix, Multipath management software may work with some storage devices but not with others. Similarly, host bus adapters on certain operating systems with certain firmware will work with some storage devices but not with others. Moreover, HBA firmware on hosts may be inadvertently upgraded by systems administrators without storage administrators' knowledge, leading to problems in the heterogeneous SAN. Detecting events in the environment and correlating them to a root cause becomes increasingly complex.

Now, the most difficult part. The storage management tools for these environments are quite immature. Often sitting out-of-band, they cannot provide both a centralized management view and the fine-grain virtualization that is needed to simplify the environment. Without the centralized management view, the complexity cannot be understood; yet without the fine-grain virtualization the complexity cannot be reduced. Consequently, the environment's operational costs are driven higher and expensive professional services knowledge is needed to create and maintain the environment. For those storage administrators entranced by Circe's call, the transformation to beast of burden is complete.

[FIGURE 2 OMITTED]

Moving Intelligence Into the Network

What is the answer? For most, they resist the call and remain with direct-attached storage or multiple homogeneous SAN islands, Unfortunately, they do not achieve any of the benefits outlined above. Their storage infrastructure is locked in a time capsule that can't evolve as IT business requirements change. This is not sustainable over the long term.