Saturday, January 04, 2014

What is SAN Fill Word?

This is snip from Brocade SAN Admin Best Practicies ...

Note: Fill Word (apply for 8 Gbps platform only)

Prior to the introduction of 8 Gb, IDLEs were used for link initialization, as well as fill words after link initialization. To help reduce electrical noise in copper-based equipment, the use of ARB (FF) instead of IDLEs was standardized. Because this aspect of the standard was published after some vendors had already begun development of 8 Gb interfaces, not all equipment can support ARB (FF). IDLEs are still used with 1, 2, and 4 Gb interfaces. To accommodate the new specifications and different vendor implementations, Brocade developed a user-selectable method to set the fill words to either IDLEs or ARB (FF). Currently, setting the fill word can be done only via the CLI command portCfgFillWord (Ex: portcfgfillword [slot/]port, mode). There are four modes:

Mode 0 - Use IDLEs in link initialization and IDLEs as fill word (default mode).
Mode 1 - Use ARB (FF) in link initialization and ARB (FF) as fill words.
Mode 2 - Use IDLEs in link initialization and ARB (FF) as fill words.
Mode 3 - Try Mode 1 first; if it fails, then try Mode 2.

Traffic outside of frame traffic is made up of fill words: IDLEs or ARB (F0) or ARB (FF). Encoding errors on fill words are generally not considered impactful. This is why you may see very high counts of enc_out (encoding outside of the frame) and not have customer traffic affected. If many fill words are lost at once, the link may lose synchronization. On standard E_Ports, primitives are set to ARB, regardless of the portcfgfillword setting when not in R_RDY mode.

The recommended best practices are:
  • Ensure that the fill word is configured to Mode 3.
  • When connecting to a HDS storage device, set to Mode 2.
  • When upgrading firmware, recheck the settings, since the fill word primitive has evolved over several Brocade FOS releases.

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