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Distance replication
The primary benefit of port buffer credits is to keep data flowing across distances. The size of the buffer credit needed on each FC port will depend on four factors:

  • The amount of data going through the port
  • The speed of the port
  • The distance between the FC ports
  • If the WAN gateway devices used provide additional buffering
Default port buffer settings on most directors will work fine without adjustment. Although the default settings range from eight on Brocade's SilkWorm 48000 to 16 on McData's i10K, these settings will work fine for most locally attached AIX, Hewlett-Packard (HP) Co., Sun Microsystems Inc. and Windows servers, and most storage arrays. When FC ports are used for distance replication, more buffer credits are generally required.

For distance replication, vendors generally recommend approximately one port buffer credit for every kilometer over a 1Gb/sec link. In most situations, you'll only need to devote a few FC ports for long-distance replication with the rest of the ports reserved for local connectivity. To provide as much flexibility as possible, vendors offer choices for how buffer credits can be configured and re-allocated among ports.

The ability to allocate buffer credits to FC ports lets users install higher port-count line cards and meet high-throughput replication and

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normal FC connectivity requirements. For example, each line card on McData's i10K has a pool of 2,746 buffer credits to draw from, with 1,373 buffer credits available per line card processor. Each of the 1,373 buffer credits may be redistributed among any of the FC ports associated with the processor. So in instances where data needs to be replicated to a site 190km away, McData recommends assigning 1,125 buffer credits to each 10Gb/sec link. By using the i10K line card that offers 24 2Gb/sec FC ports and two 10Gb/sec FC ports, each of the two 10Gb/sec ports can be allocated 1,125 buffer credits; this leaves 248 buffer credits total or 20 buffer credits for each of the remaining 2Gb/sec FC ports, which is more than enough for local FC connectivity.

Cisco is the only director vendor that allows users to increase the number of buffer credits, up to 3,500, with an optional license. The only other way to attain sufficient buffer for distance replication is to use a gateway device. Gateway appliances such as Ciena Corp.'s CN 2000 or Nortel Networks' Optical Metro 5000 series (formerly OPTera Metro 5000) offer additional buffering that enables the directors to do long-distance replication over FC when they're connected to SONET or dense wavelength division multiplexing (DWDM) networks.

This was first published in February 2006

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