The balancing process starts by placing each drive on its preferred path, if one has been assigned. It will
then balance the paths for any drives that do not have a preferred path. The command will balance paths
even if there are sessions established with the tape drives.
To balance the tape drive paths:
echo rebalance > /sys/bus/scsi/drivers/pfo/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
Procedure
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
Procedure
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 Hewlett
Packard Enterprise LTO-6 FC drives integrated into Hewlett Packard Enterprise 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
Path rotation exercise function
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