available control path drive to determine the preferred control path drive (called the ‘active’ control path
drive) before requesting a connection.
Driver operation during normal communications
The advanced path failover driver passes commands through without any command translation and with
minimal additional processing in normal operation to retain the maximum possible performance. During
normal communication the advanced path failover driver does not introduce additional commands that
would cause delays (for example, commands to determine position) during reading and writing.
Path failure detection
The advanced path failover driver uses notifications from the SCSI subsystem that report link failures
immediately following a path failure, which allows recovery to happen as quickly as possible so most
recoveries complete before the standard command timeout. In some operating systems the path failure
notification is received immediately after the failure and the failover drivers are able to perform path failure
recovery even if there are no outstanding commands. In other operating systems the advanced path
failover drivers are only notified of a path failure when a command is transmitted over that path.
Path failure recovery
Following detection of a path failure the advanced path failover driver queries a path verification feature in
the LTO tape drive to test paths until a valid path to the device is detected. The path verification feature
allows rapid detection of failed and valid paths without waiting for long timeouts or hardware-specific
notifications. After a new path has been identified the advanced path failover device driver will send a
command to the device using the new path to notify the device that a path has failed, indicate which
connection has failed, and to provide state information. Upon receipt of a notification that the path has
changed, the target device will automatically transfer all available settings and information from the failed
connection to the new connection and use the state information provided in that change notification to
synchronize the target state with the device driver state then will notify that device driver that it has
successfully synchronized state. Synchronization of the state includes any physical position changes
necessary to position the tape in the correct logical position for that state.
After receiving notification that the state is synchronized between the advanced path failover device driver
and the target device, the advanced path failover device driver can take the steps necessary to recover
any commands that were outstanding at the time of the failure. For most commands recovery is
accomplished by resending the original command.
Notifying the target device of the path change and performing the state synchronization in the target
device removes complex state recovery algorithms from the driver and removes the risk of incorrect tape
positioning during state recovery, resulting in a higher performance, lower complexity, and less risky path
failover method than a traditional driver where all recovery is performed by the driver.
Active and passive control path drives
The SCSI connection to libraries using advanced path failover is through the physical link in a tape drive.
Libraries that support advanced path failover will configure two different tape drives so that they present a
library control path (Medium Changer) device and forward commands addressed to the library control
path device on to the changer controller in the tape library. One of the tape drives will be configured as an
“active” control path drive, which means that the library control device presented by this tape drive will
accept commands such as MOVE MEDIUM for the changer device. One of the tape drives will be
configured as a “passive” control path drive, which means that the library control device presented by this
tape drive will accept device discovery commands such as INQUIRY, but will reject commands such as
MOVE MEDIUM. The advanced path failover drivers will automatically select an “active” path to the library
and will automatically reconfigure which drive is the active control path drive when reconfiguration is
necessary during failover. The tape library user interface will show which control path drive is the current
active control path drive.
14
Driver operation during normal communications
