Control Path Failover, Data Path Failover, And Load Balancing - IBM TS3500 Manual

• Model D53 to D23
Models L23 and D23 equipped with feature code 4871 (TS7700 Backend Switches mounting
hardware) must also have feature code 1950 (Power Distribution Unit), which enables dual
ac power distribution to the Fibre Channel switches as well as to the enhanced frame control
assembly.
Parent topic: Power structure of the TS3500 Tape Library

Control path failover, data path failover, and load balancing

This section discusses measures that the TS3500 Tape Library uses to control library and
data path failure, and to balance workload.
Command failures and time-outs are costly. You want your library to run smoothly and
efficiently. To ensure continued processing, libraries that are equipped with Fibre Channel
Ultrium and 3592 tape drives offer path failover and load balancing capabilities that enable
the IBM device driver to resend a command to an alternate path. The alternate path can
include another host bus adapter (HBA), Storage Area Network (SAN), or library control
path drive. The device driver initiates error recovery and continues the operation on the
alternate path without interrupting the application. Path failover and load balancing are
built-in features that are enabled by using a purchased license key.
Two types of path failover capabilities exist: control path failover (CPF) and data path
failover (DPF). Control refers to the command set that controls the library (the SCSI
Medium Changer command set on LUN 1 of the tape drives). Data refers to the command
set that carries the customer data to and from the tape drives (the SCSI-3 Stream
Commands (SSC) device on LUN 0 of the tape drives). Path failover means the same thing
in both: that is, where there is redundancy in the path from the application to the intended
target (the library accessor or the drive mechanism, respectively), the device driver
transparently fails over to another path in response to a break in the active path.
Both types of failover include host-side failover when configured with multiple HBA ports
into a switch, but CPF includes target-side failover through the control paths that are
enabled on more than one tape drive. DPF includes target-side failover for 3592 tape drives
only by using the dual-ported interface of the drive.
DPF includes load balancing of the HBAs because the channel is a data-intensive path (the
control path carries very little data, so load balancing is not an issue). The dynamic load
balancing support optimizes resources for devices that have physical connections to multiple
HBAs in the same machine. When an application opens a device that has multiple HBA paths
configured, the device driver determines which path has the HBA with the lowest usage and