Users Guide

Library sharing
The library can be congured into one or more logical libraries that can be shared by multiple
applications.
It is advantageous to be able to share a single physical library between heterogeneous or homogeneous
applications. However, some applications (and some servers) do not allow for sharing a library between
systems.
The library Management GUI provides two methods for logical library conguration.
1. A quick conguration for a simple one logical library conguration
2. An advanced conguration action for a multiple logical library conguration
Note: When any number of drives are loaded, a warning message appears when the Manage Logical
Library (Expert Mode) wizard is accessed.
The second method gives the ability to create congurations that enable the library to process commands
from multiple heterogeneous applications (such as a Windows application) and multiple homogeneous
applications (for example, the same application run by several System p servers). See “Advanced library
conguration” on page 56.
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 efciently. Path
failover capabilities allow the 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 eld upgrade. The path
failover feature is activated on the Management GUI. To order features, contact your Sales
Representative or Business Partner.
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 congured 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 congured, 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 current device driver. They are supported exclusively with
products that bear the Dell logo on the operating systems that is indicated in Table 14 on page 19
.
Table 14 on page 19 summarizes the differences between CPF, DPF, and load balancing.
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Dell EMC ML3 Tape Library: User's Guide