Impacts of device changes on the SAN

The SAN environment can shift dramatically due to device or cabling changes. The dynamic nature of the SAN can cause static definitions to fail or become unpredictable.

Device IDs that are assigned by the SAN and known to the server or storage agent can be altered due to bus resets or other environmental changes. For example, the server might know a device X as rmt0 (on AIX®), based on the original path specification to the server and original configuration of the LAN. However, some event in the SAN, for example, the addition of new device Y, causes device X to be assigned rmt1. When the server tries to access device X by using rmt0, either the access fails or the wrong target device is accessed. The server attempts to recover from changes to devices on the SAN by using device serial numbers to confirm the identity of devices it contacts.

When you define a drive or library, you have the option of specifying the serial number for that device. If you do not specify the serial number when you define the device, the server obtains the serial number when you define the path for the device. In either case, the server then has the device serial number in its database and can use it to confirm the identity of a device for operations.

When the server uses drives and libraries on a SAN, the server attempts to verify that the correct device is used. The server contacts the device by using the device name in the path that you defined for it. The server then requests the serial number from the device, and compares that serial number with the serial number that is stored in the server database for that device.

If the serial number does not match, the server begins the process of discovery on the SAN, attempting to find the device with the matching serial number. If the server finds the device with the matching serial number, it corrects the definition of the path in the server's database by updating the device name in that path. The server issues a message with information about the change that is made to the device. Then, the server proceeds to use the device.