Driver operation during device open, Opening a connection to a library control path – HP StoreEver MSL6480 Tape Library User Manual
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Table 1 Example path list
Example Logical Unit Worldwide
Identifier
Example SCSI address
Port
Addressed Logical Unit
SAN
50:01:10:a0:00:00:00:01
ID 1 LUN 0
Port A
Tape drive 1
1
50:01:10:a0:00:00:00:02
ID 1 LUN 1
Port A
Library Controller
1
50:01:10:a0:00:00:00:01
ID 2 LUN 0
Port B
Tape drive 1
2
50:01:10:a0:00:00:00:02
ID 2 LUN 1
Port B
Library Controller
2
50:01:10:a0:00:00:00:03
ID 3 LUN 0
Port A
Tape drive 2
1
50:01:10:a0:00:00:00:02
ID 3 LUN 1
Port A
Library Controller
1
50:01:10:a0:00:00:00:03
ID 4 LUN 0
Port B
Tape drive 2
2
50:01:10:a0:00:00:00:02
ID 4 LUN 1
Port B
Library Controller
2
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 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.
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 available control path drive to determine the
preferred control path drive (called the ‘active’ control path drive) before requesting a connection.
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Overview