candidate for failover. If the device is a candidate for failover, the driver determines the serial number
(SN) or worldwide name (WWN) of the device. If the device is capable of advanced path failover, the
driver determines whether failover is enabled for the device. If the device supports advanced failover and
failover is enabled, the driver masks all duplicate paths and presents a single path to application clients. If
the device does not support advanced failover or if failover is not enabled, the driver will not modify the
presentation of device paths to applications.
To improve throughput for multiple tape drives, the driver attempts to balance the number of tape drives
on each HBA path. When opening a tape drive device file, the driver selects the path with the fewest
number of open tape drive device files. The load balancing algorithm does not take path speed into
account when selecting a path. If the load balancing algorithm cannot select an appropriate path, it will
select the last known good path or another available path. If a “preferred” path is configured, this path will
take precedence over the load balancing method. (See
on page 56.)
Load balancing is only applied when a tape drive device file is opened by the first thread; the paths are
not reevaluated at other times. For example:
• The failover algorithms do not consider other tape drive paths when selecting a path.
• The path is not changed if another thread already has the tape drive device file open.
• The path is not changed during data transfers.
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Installing and using Linux advanced path failover drivers
