Control path failover, data path failover, and load balancing

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The path failover feature of the tape library 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. To ensure continued processing, libraries that are equipped with Fibre Channel LTO offer path failover and load balancing capabilities. These 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.

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 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 the dual-ported tape drives that are supported by the tape library.

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 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.
Table 1. Differences between CPF and DPF
Characteristic CPF DPF and Load Balancing for LTO tape drives
Device type SMC1 SSC2
LUN3 LUN 1 LUN 0
Host-side failover Yes Yes
Target-side failover Yes Yes
IBM device driver required Yes Yes
Operating systems supported AIX®, SuSE Linux®, Red Hat Enterprise Linux, Solaris, Windows, HP-UX, Asian UX AIX, SuSE Linux, Red Hat Enterprise Linux, Solaris, Windows4(DPF only), Asian UX
Order feature to obtain license key No No
SCSI attachment supported Yes No
Fibre Channel attachment supported Yes Yes
Notes:
  1. SMC = SCSI-3 Medium Changer Specification (library)
  2. SSC = SCSI-3 Stream Commands (drive)
  3. LUN = logical unit number
  4. Load balancing is not supported on Windows

For more information about using these features, see the IBM Tape Device Drivers Installation and User's Guide.