SAN
Addressed Logical
Unit
Port
Example SCSI
address
Example Logical Unit
Worldwide Identifier
2
Tape drive 1
Port B
ID 2 LUN 0
50:01:10:a0:00:00:00:
01
2
Library Controller
Port B
ID 2 LUN 1
50:01:10:a0:00:00:00:
02
1
Tape drive 2
Port A
ID 3 LUN 0
50:01:10:a0:00:00:00:
03
1
Library Controller
Port A
ID 3 LUN 1
50:01:10:a0:00:00:00:
02
2
Tape drive 2
Port B
ID 4 LUN 0
50:01:10:a0:00:00:00:
03
2
Library Controller
Port B
ID 4 LUN 1
50:01:10:a0:00:00:00:
02
In this example the server is able to evaluate the worldwide identifiers to determine that there are two
paths to each of two different tape drives and four paths to a library controller.
In a typical multi-path configuration all of the paths for each device are available to the application which
must resolve redundant paths and choose a preferred path.
When advanced path failover drivers are installed on the server, the drivers will query each device for
support of advanced path failover and if the device supports advanced path failover, the drivers will select
a preferred path for each device and establish a connection using the preferred path. Only one path will
be visible to the application.
Driver operation during device open
When an application requests that a connection to a device be opened, the failover driver will connect to
the selected device, and if that device supports advanced path failover, will negotiate for a connection (a
nexus using INCITS/T10 standards terminology) to that device. Computers running Windows will open a
connection to each device when the server is booted. Computers running HP-UX and Linux don't open a
connection to the device until an application opens the device for reading and/or writing.
Each device supports up to 32 simultaneous failover connections. If a failover connection is available, a
connection between the host driver and the device is created. If no failover connection is available, the
connection that has been idle for the longest time is closed and then the new connection is created. The
server connected to the terminated idle session is notified that the failover connection has been
terminated and the advanced path failover driver on that system will automatically create a new
connection the next time a command is sent to the device.
NOTE:
Systems using advanced path failover should be designed with 32 or fewer active hosts per device
for optimal performance. Hosts should be configured so that they do not send unnecessary polling
commands to the device.
Driver operation while opening a connection to a library control path
In a system configured for advanced path failover at least two tape drives should be configured to provide
a path to the library controller. For additional redundancy, the ESL G3 library allows configuring more than
one standby control path drive. Some library state information is retained in the drive hosting the library
controller so each server that requests a connection to the library controller is required to create a
connection using a path through the same hosting drive. The library controller provides a method for the
administrator to select the preferred control path drive. The advanced path failover driver will query each
Driver operation during device open
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