Knowledge Base

LUN (or device) presentation configuration (also known as "device masking configuration") on the EMC Symmetrix is stored in a special database on the Symmetrix known as the VCM (or Volume Control Management) database. This database stores information such as which LUNs or devices are presented to which hosts and through which FA director ports. The current LUN presentation information is cached into the FA memory, and this information is part of what is called the FA's "memory bitmaps." When the SAN administrator changes the LUN representation configuration, this change is stored only in the VCM database and will not be in the current FA's memory bitmaps. As far as the ESX Server hosts are concerned, newly presented LUNs on the FAs will only be visible after the FA's memory bitmaps are refreshed and up to date. In other words, the change will not take effect on the FAs until it is made "active" -- that is, by updating the FA's memory bitmaps.

There are two methods you can use to refresh FA memory bitmaps.

• The command must be run from within a management host. A management host is any host that has the "symm api" agent installed on it.
  
  
• The management host must have access to a Symmetrix gatekeeper LUN.
  
  Note: By default, ESX Server hosts are not management hosts, although they do see gatekeeper LUNs. ESX Server hosts do not need to be management hosts.

• Get the name of the fibre channel module:
  vmkload_mod -l
• Unload the module:
  vmkload_mod -u
• Load the modules:
  vmkload_mod <modulenam>
  Note: Loading the module may take a few moments.

For more information on using either of these methods, refer to EMC's documentation on using ECC, Solution Enabler Administration, and Symmetrix Device Masking Administration.