Full Balancing algorithm
When a device file is opened to a tape drive, the driver determines the number of open device files to
tape drives using the HBA of the last known good path. The driver selects a path on an HBA that is in use
by the fewest number of open tape drive device files. It will change paths even if there has not been a
path failure event.
Using the Full Balancing setting can result in frequent path changes on a busy system that has multiple
drives connected to multiple HBAs. Each path change can cause a path failover alert from the library. The
library is unable to distinguish a path change that is the result of a path failure from a path change to
optimize data throughput, so the library will issue an alert in both cases. If alerts are enabled in the library
and path change alerts are undesirable, the Less Balancing algorithm might be a better choice.
Less Balancing algorithm
When a device file is opened to a tape drive, the driver determines the number of other tape drives using
the HBA of the last known good path. The driver selects a path on an HBA that is in use by the fewest
number of other tape drives. The driver will not select a different path if there is a failover session already
established on that path to the tape drive.
If a path is lost, the driver knows that an alert will result anyway, so it will attempt to balance traffic among
the remaining paths at that time.
This algorithm will not be as effective as Full Balancing in optimizing data throughput, but will minimize
the frequency of library path failure alerts.
Setting the load balancing level
The level of load balancing aggressiveness can be set on the fly. This setting is volatile across reboot.
To set Full Balancing:
echo 2 > /sys/module/pfo/parameters/pfo_balance
To set Less Balancing:
echo 1 > /sys/module/pfo/parameters/pfo_balance
To stop load balancing:
echo 0 > /sys/module/pfo/parameters/pfo_balance
Setting the load balancing reboot option
The default load balancing algorithm is Less Balancing.
To set Full Balancing at all reboots:
echo "options pfo pfo_balance=2" > /etc/modprobe.d/pfo-balance.conf
To not use load balancing at all reboots:
echo "options pfo pfo_balance=0" > /etc/modprobe.d/pfo-balance.conf
NOTE:
For a one-time path balancing process, see
on page 56.
Command line user interface
The advanced path failover drivers support several configuration and diagnostic functions through a
command line interface. Most of the functions are accessed by directing commands to a specific path.
The following example shows the command for a specific path and the commands may be directed at any
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Full Balancing algorithm
