Question & Answer
Question
What are PIDs? When do they Change? How to avoid Dynamic Tracking changes ?
Cause
PID/N_PIDs, N_Port ids, scsi_ids(in AIX),Fabric Addresses or FCIDs are used by the routing and zoning services in FC fabrics to identify node ports / N_Ports in a fibre channel fabric. Every device when plugged into a fabric is assigned a unique PID or N_Port id.
There are various reasons why the N_Port ID might. Following are some of the various scenarios which result in N_Port id changes:
1. Anytime a FC cable is moved to a different port or a different switch at either the initiator or target side.
2. For NPIV clients, the N_Port id changes when its logged out and logged back in to the fabric, if WWN based port binding is not implemented on the san switch.
NPIV (Virtual) Fibre Adapters may see an N_Port id change in the follow scenarios:
1. When a virtual adapter is logged into the fabric for the first time, the switch assigns a unique N_Port id.
2. If a virtual adapter is logged out of the fabric for any reason and when it logs back into the fabric, it is assigned a different N_Port id in many cases.
3. Upon getting the new N_Port id, AIX will go through a Dynamic Tracking event updating the driver with the new FCID.
4. If possible, It is advisable to avoid Dynamic Tracking changes which update structures in the driver to use the new FCID, in order to prevent delays/error recovery as the driver updates its internal structures. This is particularly true for the simple case of rebooting an NPIV client LPAR, where no real cable moves have been performed.
Answer
In case of virtual adapters, Dynamic Tracking upon reboot of the LPAR can be avoided by enabling a switch setting which by default is disabled.
The setting is called WWN Based Persistent PID and needs to be enabled on the switches.
This setting applicable to both Broacde and Cisco fabric switches.
When a physical initiator/HBA or target port's N_Port id changes (i.e., there actually HAS been a cable move), there is no way to avoid Dynamic Tracking events.
Summary:
Dynamic Tracking changes are unavoidable when moving FC cables to a different switch or to a different switch port.
However, Dynamic Tracking is avoidable for NPIV clients (at boot time) if WWN based Persistent PID is enabled .
Was this topic helpful?
Document Information
Modified date:
17 June 2018
UID
isg3T1024829