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|Decoding naming conventions|
|For a newcomer to the SAN environment, HBA
naming conventions can quickly become confusing. For example consider the 1Gb and 2Gb naming
convention. Paul Spagnolo, an IBM field engineer, says it's critical to distinguish between gigabit
(Gb) and gigabyte (GB). Fibre Channel (FC) protocols, he explains, measure their performance in
gigabits, not gigabytes. Yet FC speeds are referred to in megabyte (MB)/sec. Why?
The current FC transfer speeds of 100MB/s and 200MB/s are arrived at in the following manner: FC uses a common encoding scheme call 8B/10B encoding. In this way, two extra bits are added to the 8-bit packet making the packet 10-bits in length. The 100MB or 200MB performance numbers are arrived at by dividing the 1Gb and 2Gb speeds by the 10-bit packet. Half duplex 1GB with the 8B/10B encoding scheme now becomes 100MB and full duplex 1Gb FC links could potentially support 200MB, if the application could support and sustain that speed.
While this may seem like a small point, this knowledge comes to bear when sizing performance throughput on the SAN. If one begins with the false design assumption that you have 1GB or 2GB throughput as opposed to 1Gb or 2Gb, you may be setting yourself up for bottlenecks to appear in your SAN design.
A more confusing concept in the SAN space is the naming schema used by HBAs, the World Wide Name (WWN). While it may sound confusing, it corresponds almost exactly to the Media Access Control (MAC) address found on every network interface card (NIC). The WWN is a hard-coded 16 character hexadecimal address that's unique to--and appears on--each HBA which is similar to the MAC address that uniquely identifies each NIC.
So why do you hear so much about WWN and so little about MAC? Simply because SAN technology has not yet matured to the point that LAN/WAN technology has. What happened in the LAN/WAN space is network protocols, and specifically TCP/IP, overlaid and virtualized the embedded MAC address to allow routing and a more intelligent, manageable network design. The MAC address still exists but TCP/IP masks the underlying hard coded MAC address and makes it transparent to the end user. While this sort of technology may emerge in the SAN space, it has not yet occurred. So the WWN term--when used--still does more to confuse the topic of SANs than clarify it.
So, what's the best HBA for your environment? The first key area of functionality where HBAs diverge is in their current ability to provide dynamic load balancing across multiple HBAs. One of the key areas of concern within SANs is high availability. To achieve high-availability requirements, redundant paths to the storage are deployed using either two HBAs or two HBA connections in a server.
Until recently, the only way to dynamically load balance the storage traffic across HBAs was to use a proprietary HBA driver supplied by the storage array vendor. While this isn't a problem if the server only utilizes storage from only one storage array vendor, it becomes an issue if the server has access to storage from vendors of different storage arrays concurrently.
There's one critical question regarding HBA load-balancing capabilities: Is their dynamic load-balancing driver software designed to support an Active-Active or an Active-Passive configuration? In an Active-Active configuration, the I/O traffic is equally shared down both paths into the SAN. In an Active-Passive configuration, one card-the Active card-carries the brunt of the I/O traffic to the SAN while the passive card waits for either busy times or the other card to fail before it carries traffic. Although not every HBA vendor currently offers this feature, Emulex, JNI, and LSI Logic offer this functionality in an Active-Active configuration. Emulex released this functionality in late October 2002 in its MultiPulse technology. In addition to load balancing, the MultiPulse technology also offers instantaneous rerouting of traffic around a failed element and protection of application availability. JNI offers this function with their FibreStar and Zentai line of HBAs and LSI Logic's driver has the unique ability to actively load balance across as many of its cards as the host system can populate without restrictions.
Atto Technology Inc., Amherst, NY, plans to offer this feature in Q1 of 2003 though they're still debating between an Active-Passive or Active-Active deployment. QLogic offers the Active-Passive option as the default driver install though the Active-Active driver is available as an optional driver install.
The result of deploying this technology from the HBA vendor is that it may help to reduce or eliminate your reliance on your a single storage array vendor for your storage and their proprietary driver to deploy a dynamic load-balancing solution on the SAN.
Yet deploying the vendor's HBA load-balancing solution has at least two downsides. One, it may result in some loss of functionality that a storage array vendor's load-balancing driver may provide, such as Oracle database awareness on the storage array. The other is the lack of standards.
Another feature closely associated with dynamic load balancing is failover functionality. It differs slightly from dynamic load balancing because if one HBA or its path to the storage should fail, the other HBA transparently takes over without interruption to the application. This functionality is a requirement in any high availability environment, and should be considered a prerequisite prior to deploying any SAN solution in these environments. Again, when choosing an HBA for this functionality, exercise some judgment.
This was first published in December 2002