HP StoreEver MSL6480 Tape Library - Manual (Page 46)

echo prefer=8:0:1:0 > /sys/class/pfo/pfo5/ctrl
You can disable the configuration of a preferred path by creating an empty prefer entry in the
crtl
file. For example:
echo prefer > /sys/class/pfo/pfo5/ctrl
Path rotation exercise function
Path rotation is a test function that will cause the driver to change the path it uses each time the
device file is closed and reopened.
To cause the path to change with every open() of the /dev file, set rotate=1. For example:
echo rotate=1 > /sys/bus/scsi/drivers/pfo/ctrl
To stop path rotation, set rotate=0. For example:
echo rotate=0 > /sys/bus/scsi/drivers/pfo/ctrl
Enabling advanced path failover on a device while the driver is running
If a device has any advanced path failover feature disabled when advanced path failover is
enabled, the device will reset itself, removing the old /dev file. When the device comes back up,
it will be recognized as an advanced path failover device. It will then operate normally as an
advanced path failover device. It may not have the same /dev file name as before the switch.
Disabling advanced path failover on a device while the driver is running
Disabling advanced path failover while a device is running is not recommended because the paths
will not be cleanly removed and reassociated. If advanced path failover is disabled on any device,
the Linux server will need to be rebooted. When possible, power down the Linux server cleanly,
and then disable advanced path failover on the device, and then boot the Linux server.
Linux driver theory of operation
The Linux advanced path failover drivers support data path failover and control path failover on
HP LTO-6 FC drives integrated into HP libraries. The failover functionality is provided by the pfo
driver and modifications are made to the standard st and sg drivers to call into the pfo driver
if a device supports advanced path failover. The modified st and sg drivers are called stmp and
sgmp
.
The failover device driver must identify devices accessible over the different paths available and
associate paths with devices so that the driver can mask duplicate paths and present a single path
to an application.
As devices are discovered by the operating system and the device information is passed into the
driver for initialization, the driver determines if the device is a tape or library device, and if so,
whether it is a 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.
46 Installing and using Linux advanced path failover drivers