1. Primary data path
2. Secondary data path
3. Host connection
4. Ultrium dual port Fibre Channel tape drive
Basic control path failover details
Library basic control path port failover (see
Figure 4: Control path port failover example configuration
on page 11) is configured with one drive hosting the active logical link to the library controller and a
second drive configured as the passive logical link to the library controller. The library connections share
physical links with the drives while the library has its own identity on the SAN and will appear as a unique
device. Unlike the typical bridged library controller configuration shown in
on page 7, basic control path failover uses a Fibre Channel technology
called NPIV, which allows the library to appear on the SAN as a new device with its own worldwide name
instead of appearing as a logical unit behind the tape drive. Connecting as an independent device makes
it possible to change the library controller SAN connection without affecting the drive connection.
If basic data path port failover is also configured, the library control path follows the data path on single
link failures.
With basic control path failover, the ESL G3 library monitors the drive that is hosting the library controller
and the ports on the drive. The library receives notification from the drive of any interface change events
and can quickly detect whether that change indicates a fault. The library also monitors the drive and can
detect when a drive has been removed or otherwise becomes inaccessible. Upon detecting a fault that
would cause loss of communication with the library controller, the library automatically moves the library
control path identity (world-wide names) and all settings (such as reservations and prevent/allow settings)
over to the alternate control path drive and activates the control path on the alternate drive. When
properly configured, the change is minimally disruptive to the host and does not require any configuration
changes on the host or in the backup application.
NOTE:
The MSL libraries monitor drive removal events and if the active control path drive is removed, the
library will automatically move the control path to the standby drive. However, if the control path
drive is not removed but SAN connectively is lost, the administrator might need to use the library
remote management interface to move the control path to the standby drive.
If no library commands are sent while the port is being reconfigured, the port change happens with
virtually no disruption to the SAN. Commands sent while the port is being reconfigured might be
completed but cannot report the command status to the application so the application will receive an
error. Applications that retry commands are able to use the library following reconfiguration of the port.
Applications that do not retry can be restarted remotely without making any hardware configuration
changes.
10
Basic control path failover details
