Specifications

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Summary of Best Practices for Storage Area Networks

mechanisms in Fibre Channel for dealing with congestion in the fabric itself are not

very effective. The problems caused by fabric congestion can range anywhere from

dramatically slow response time all the way to storage access loss. These issues

are common with all high-bandwidth SAN devices and are inherent in Fibre Channel;

they are not unique to the SVC.

When an Ethernet network becomes congested, the Ethernet switches simply

discard frames for which there is no room. When a Fibre Channel network becomes

congested, the FC switches will instead stop accepting additional frames until the

congestion clears, in addition to occasionally dropping frames. This congestion

quickly moves “upstream” and clogs the end devices (such as the SVC) from

communicating anywhere, not just the congested links. (This is referred to in the

industry as head-of-line blocking.) This could have the result that your SVC will be

unable to communicate to your disk arrays or mirror write cache because you have a

single congested link leading to an edge switch.

All ports in an SVC cluster should be connected to the same dual-switch fabric as all

storage devices the SVC is expected to access. Conversely, storage traffic and

inter-node traffic should never transit an ISL, except during migration and some

SVC Stretched Cluster scenarios. Due to the nature of Fibre Channel, it is

extremely important to avoid Inter Switch Link (ISL) congestion. While Fibre

Channel (and the SVC) can, under most circumstances handle a host or storage

array becoming overloaded, the mechanisms in Fibre Channel for dealing with

congestion in the fabric itself are not very effective.

The problems caused by fabric congestion can range anywhere from dramatically

slow response time all the way to storage access loss. These issues are common

with all high-bandwidth SAN devices and are inherent in Fibre Channel; they are not

unique to the SVC. High-bandwidth-utilization servers, (such as tape backup

servers) should also be on the same switch as the SVC. Putting them on a separate

switch can cause unexpected SAN congestion problems. Putting a high-bandwidth