Control path failover, Data path failover, and load balancing
The path failover feature ensures the use of a redundant communication path when the primary path fails.
Command failures and time outs are costly. You want your library to run smoothly and efficiently. Path failover capabilities allow 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. The path failover feature can be ordered from the factory, or you can order it as a field upgrade. The path failover feature (FC 1682) is activated on the Management GUI. To order features, contact your IBM® Sales Representative or Business Partner. See Optional features.
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. Path failover is where redundancy is in the path from the application to the intended target (the library accessor or the drive mechanism), 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 the dual-ported tape drives that are supported by the library.
DPF includes load balancing of the HBAs because the channel is a data-intensive path (the control path carries 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 where multiple HBA paths are configured, the device driver determines which path has the HBA with the lowest usage and assigns that path to the application. When another application opens a different device with multiple HBA paths, the device driver again determines the path with the lowest HBA usage and assigns that path to the second application. The device driver updates the usage on the HBA assigned to the application when the device is closed. Dynamic load balancing uses all HBAs whenever possible and balances the load between them to optimize the resources in the machine.
Both CPF and DPF require the use of the IBM device driver. They are supported exclusively with products that bear the IBM logo on the operating systems that is indicated in Table 1.
Table 1 summarizes the differences between CPF, DPF, and load balancing.
| Characteristic | CPF | DPF and Load Balancing |
|---|---|---|
| Device type | SMC1 | SSC2 |
| LUN3 | LUN 1 | LUN 0 |
| Host-side failover | Yes | Yes6 |
| Target-side failover | Yes | Yes6 |
| IBM Device driver required | Yes | Yes |
| Supported operating systems4 | AIX®, SuSE Linux, Red Hat Enterprise Linux, Solaris, Windows | AIX, SuSE Linux, Red Hat Enterprise Linux, Solaris, Windows5 (DPF only) |
| Order feature to obtain license | Yes | Yes |
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Notes:
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For information about using these features, see the IBM Tape Device Drivers Installation and User's Guide (GA32-0565).