Asymmetrical SANs

Asymmetrical SAN, or out-of-band, can be an efficient use of resources for your SAN implementation.

Asymmetrical SANs
Rick Cook

Most Fibre Channel SAN architectures use a separate path, usually over the LAN, to convey control signals, requests for data and other metadata between the server and the SAN. Thus, "LAN- free" operation of a SAN usually isn't completely LAN free, although the SAN enormously reduces the load on the LAN.

This kind of architecture, called "out of band" or an "asymmetrical SAN" seems paradoxical because of the SAN's high bandwidth, but it actually represents an efficient use of resources. For one thing it allows the SAN to be optimized for data blocks, which are usually much longer than the messages associated with managing the SAN. For another it allows the Fibre Channel loop to be managed even if the loop itself is down.

But an asymmetrical architecture does complicate management of functions such as SAN security. In effect, the metadata channel has to be secured separately from the data channel, which adds complexity.

The terms are also used to refer to the different SAN administration architectures that manage the SAN overall. Unlike Fibre Channel SAN control information, which is almost always out-of-band, SAN administration software uses one of three approaches only one of which is sometimes out-of-band. In host-based administration, the software resides on the application servers which need to access the SAN. In storage-based architectures, the software is on the storage platform. Network-based administration software may be either in-band or out-of-band, depending on the path used for metadata.

IBM discusses in-band and out-of-band management in its Redbook "Introduction to SAN", which is available on the IBM Redbook web site at www.redbooks.ibm.com. Vixel also discusses the concept in a white paper titled "The Need For Storage Virtualization" at www.vixel.com.


Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

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Related Book

Fibre Channel for SANs
Author : Alan Frederic Benner
Publisher : McGraw-Hill
Published : Mar 2001
Summary :
PRACTICAL ROADMAP FOR DESIGNING AND DEPLOYING A SANFibre Channel has come into its own as the defining network architecture for Storage Area Networks (SANs), which are proving critical for managing the volume and complexity of data generated by Internet-era applications. Fibre Channel for SANs, by Dr. Alan F. Benner, shows you how Fibre Channel works, how it integrates with other protocols and systems, and how to implement it to create a SAN for fast access to mass storage. It walks you through the ANSI standard's 5 levels, from the physical transmission level through interfaces to upper layer protocols, and demonstrates mapping SCSI and IP over Fibre Channel. You get a wealth of timesaving illustrations and practical suggestions for troubleshooting.


This was first published in June 2001

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