Consolidate storage without losing control - Security

Computer Technology Review, Sept, 2003 by Eric Blonda

The storage area network (SAN) is quickly evolving from a connectivity-only "black cloud" to a strategic opportunity to add value from the proposal stage through the life of the SAN. With the recent momentum behind trends such as storage virtualization and clustering, in which storage is virtually pooled together and host central processing units (CPUs) are banked into large clusters, hidden challenges to application performance are arising, along with new tools to address these challenges. At the same time, new levels of network intelligence are arriving in the storage network and revealing the areas of congestion lurking in every SAN. Value-added resellers (VARs) and storage integrators that can ride these trends and build connection-level intelligence into the storage network will be in a position to surpass lackluster storage implementations and offer more strategic solutions that not only support application performance but open the door to new uses for data in demanding, high-availability applications, such as collaborative enterprise software.

Booming Despite the Bust

While other technology markets take a "sit tight" approach, those of us in the SAN business are enjoying a relatively aggressive market, where investments in networked storage are translating directly into corporate advantages. Where customers are spending on technology, there's a good chance the motivation is to accommodate ever-expanding storage and related connectivity and management. As the migration from direct-attached storage to network-attached storage continues--as well as the expansion of existing SANs--challenges are arising related to size and scope, marked by a shift from deployment-oriented to production-oriented issues.

Now that SANs are becoming widely deployed, the challenge is how to make them scale, meet a broader set of requirements, and continue to improve on the initial consolidation and utilization ROI. One of the most realistic ways to measure a SAN's success is to gauge the day-to-day performance of the application(s) that the SAN supports. Most storage administrators are bound (albeit sometimes quite loosely) to the availability and performance of the business-level application and/or service that the SAN hosts. In the data center, SAN performance is typically expressed through a number of metrics taken directly from the SAN devices themselves, which translates roughly to the overall effectiveness of application-level support. SAN device performance metrics may come from the host, such as CPU utilization, from the storage array, such as disk-seek times and from the network, such as port utilization statistics. Traditionally, much more administrator time is spent evaluating the performance at the host and array, where much more tangible metrics are available, while the network has yet to be considered, either a suspect for degradation or as an opportunity for overall improvement. The main reason for this is the limited amount of useful network performance information available in most SANs, and the false impression that a network has latent bandwidth when it is in fact congested, degrading application-level performance.

New Focus on Congestion

All networks are inherently susceptible to congestion. When the purpose of a network is to allow for the communication among devices that share resources, this in itself presents the opportunity for congestion. This is especially the case in SANs, where there are typically many hosts (or initiators) communicating with relatively fewer storage ports (targets). Many are under the false impression that a network built with a switch or switches that are "non-blocking" with the ability to service all ports at line rate, will never be the culprit when overall SAN performance falters. But no amount of raw switch performance can overcome the fundamental contradiction that arises from two initiators requiring access to the same target. The problem of network congestion is only now becoming more apparent, as SANs continue to stretch their limits to scale and virtualization and blade-server computing become more popular. As storage resources are virtually pooled together and CPUs clustered, it will seem that all resources are virtually shared, aggravating the likelihood of congestion. Enterprise Storage Group founder and senior analyst Steve Duplessie has said that, "Traffic congestion is not talked about enough in the storage network market, but it's a very ugly secret that will rear its head at the worst possible time as networks get bigger."

Most network protocols, including Fibre Channel, use standards-based methods for media access and the transmission of information among systems. Fibre Channel protocol, the dominant in storage network fabrics today, uses a very carefully architected system of fabric device communications and credit-based transmissions in order to ensure network stability and traffic integrity. Unlike Ethernet/IP networks, when presented with the potential for congestion, particularly in the new "any-to-any" SAN model, with more traffic from inbound ports (i.e. initiators) than a destination port (i.e. target) can handle, Fibre Channel switches do not drop frames. Instead, the network uses a "back-off" technique to squelch all inbound traffic bound for the same destination to a level where the target can accommodate all requests.