HP StoreEver MSL6480 Tape Library - Manual (Page 13)

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
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