Overview of volume failover

Unlike configuration failover, volume failover is implemented in many forms and in many places throughout FMS. Implemented within FSC clients FSC clients use only the assignedfsc list in the fmsmaster_fscid.xml file to determine the initial FSC destination of data requests from the client. This list is derived exclusively from the priority= attributes of the elements in the client’s assignedfsc list. Implemented within FCC clients FCC clients use a variety of information from the FSC configuration to determine the initial FSC destination of a data request. These include, in order of precedence: If assignment mode="parentfsc" is specified in the fcc.xml file: The parentfsc list in the fcc.xml file is treated as the actual assignedfsc list. The assignedfsc list returned by the configuration server FSC is required but largely ignored. The FCC_EnableDirectFSCRouting element is automatically set to false, regardless of any actual settings in the fcc.xml or fmsmaster_fscid.xml files. The directfscroutes list returned by the configuration server FSC is also ignored. In this mode, the configuration failover settings are also used for volume failover at the FCC client (by design). The assignment mode="parentfsc" setting only affects FCC selection of the initial FSC server; it does not affect volume failover features at intermediate FSCs. If the FCC_TransientFileFSCSource parameter is set to ticketuri (the default setting), the URI in a transient file ticket takes precedence. FMS tickets for resource types1 other than transientvolume do not have a URI embedded in the ticket. If the FCC_EnableDirectFSCRouting parameter is set to true (the default setting), and the resource element is served within the same fscgroup element as the client’s primary assignedfsc element, the request is routed to the direct FSCs that serve it in order of the priority= attributes of the direct FSC routes. The priorities of direct FSC routes are determined by the priority= attributes of the resource and filestoregroup elements of the fsc and loadbalancer elements in the fmsmaster_fscid.xml file and delivered to the FCC with the assignedfsc list on FCC initialization. If the SiteID element (FMS-enterprise-ID) of the FMS ticket is listed in the site list of the fcc.xml configuration file, the request is sent to the assignedfsc list specific to that client, obtained from the parentfsc list specified in the corresponding site element of the fcc.xml file (using configuration failover). If none of these situations apply, or all of the attempts fail in a manner indicating appropriate failover, the request is sent to the assignedfsc list specific to that client, obtained from the default parentfsc list (not in a site element) listed in the fcc.xml file. The priority order of the assignedfsc list is determined by the priority= attributes of the assignedfsc elements of clientmap elements in the fmsmaster_fscid.xml configuration file obtained from a default parentfsc. Implemented within intermediate FSCs The bulk of the volume failover implementation is in the routing code of intermediate FSCs, which route FMS requests to other FSC servers. Intermediate FSCs use priority= attributes on a number of other elements in the fmsmaster_fscid.xml file to implement volume failover as a part of FMS message routing. These elements include: The resource and filestoregroup elements of fsc and loadbalancer elements, which apply most directly to the selection of target resource servers for FMS routing. The entryfsc elements of fscgroup elements, particularly when the entryfsc element refers directly to volume servers. The exitfsc elements of fscgroup elements, particularly when the exitfsc element refers directly to volume servers. The fscimport elements of defaultfscimport elements, particularly when the fscimport elements refer directly to volume servers in the fmsmaster_fscid.xml configuration file of the external fmsenterprise element. FMS uses client-proximity routing. Requests are routed by the first FSC to receive a request for a resource that it can neither fulfill from cache nor serve directly from volume. This initial FSC routes the request through the FMS network to another FSC that serves the requested resource. Warning It is very important that all FSC servers within the same FMS system share identical fmsmaster_fscid.xml file information. If an FSC server has an inaccurate configuration, it is prone to routing errors, which can cause user transaction failure. 1 The list of resource elements includes volume, transientvolume, logvolume, and accesson.