Sane strategies for SAN growth

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Five years ago, the first Fibre Channel (FC) storage area networks (SANs) began to be deployed in production environments. At that time, the connectivity options were few, consisting primarily of eight- or 16-port departmental switches and 32-port director-class switches. Because these early SANs served primarily as SCSI-interconnect replacements in environments connecting a relatively small number of servers and devices, this limited expandability wasn't a major problem.

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How big is too big?
Is there a point where the benefits of networked storage begin to diminish because of the liability of having all devices interconnected? As SAN node counts continue to grow, some companies have decided not to connect all devices to a single fabric. Unfortunately, they often arrive at this conclusion after experiencing a SAN event that has caused a major interruption. Even with redundant fabrics, certain component failures have the ability to disrupt I/O, causing applications to crash.

The benefits of a large interconnected SAN include:
  • Greater manageability
  • The ability to allocate storage as needed
  • More flexibility with backup resources
  • Ease of replication and migration of data
The downsides of a large-scale SAN environment include:
  • Greater potential for large-scale outages
  • Reduced local administrative control
New functionality such as autonomous regions and authoritative domains will help to mitigate these issues, as will better management tools and greater user experience with the technology.

If history is any guide, the trend will be toward larger enterprise storage networks. In five years, this may be a moot point, but for the foreseeable future, the benefits and risks must be weighed for each environment.

These initial SANs validated the touted benefits of networked storage. As the growth in the number of deployed ports and switches attests, most users have made SANs an integral part of their storage strategy. Benefits of SAN technology are derived from these three key advantages relative to the previous parallel SCSI technology:

  1. Enabling of increased distances for connectivity
  2. Ease of modification and expansion
  3. Support for a relatively large numbers of devices
These SAN benefits have led users to increase the size of their networked storage environments. Initially, expanding a small fabric is relatively easy, involving little more than adding a switch and interconnecting a couple of inter-switch links (ISLs). This is a problem-free process, assuming ports are available and similar switches from the same vendor are used. But as port counts increase and fabric meshes become more complex, some significant challenges may be encountered.

Users typically begin to encounter greater challenges when they attempt to grow their SAN environment by two times or greater, or when they consider integrating newer products to interoperate within their existing infrastructure. Two fundamental issues are managing expansion in a nondisruptive manner, and ensuring that the resulting SAN will perform as expected. Among the variety of problems commonly encountered when growing a SAN are:

  • Port availability for ISLs
  • Planning for access requirements to avoid excessive hop count
  • ISL saturation and imbalance
  • Conflicting host bus adapter (HBA) capabilities and vendor support in heterogeneous environments
  • Disruption to the production environment during transition to expanded design
  • Management and security constraints
  • Zoning complexities and potential for errors affecting the entire SAN
Some of these problems reflect the immaturity of the technology and are being addressed by vendors. Improved management capabilities and higher capacity switches help to alleviate some issues and are discussed later on in this article. Other problems relate to the overall architecture and design, which is essentially the age-old problem of understanding the requirements and building to meet those needs.

So what are the design goals to consider? A scalable SAN design has these four traits: the ability to add additional hosts and storage/tape devices without reconfiguring ISLs; the ability to increase port count without increasing hop count; the ability to add nodes with minimal placement criteria and lastly, the well thought-out application of fault isolation technologies, such as autonomous regions and zoning, which prevent a single event from disrupting an entire SAN.

This was first published in November 2003

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