Virtual TapeServer SCSI-to-FC Adapter User Manual

4-4
Chapter 4: Addressing and Logical Unit Management
For a host connected to an FC port, a map is a table of LUNs where each entry in the table is either
empty or contains device address information needed to route commands to the appropriate
device.
Each port on the VT5900-SA has a set of maps which include both user defined maps and a few
special predefined maps.
There are currently three special predefined maps: Indexed, Auto Assigned, and SCC maps. Until
a user configures the VT5900-SA otherwise, the default setting is Auto Assigned.
The Indexed map is initially empty but can be modified by the user.
The Auto Assigned map is built dynamically and contains all the devices found during discovery.
This map will change automatically any time discovery finds a change in the devices attached to
the VT5900-SA. This map can be displayed but cannot be modified directly by the user.
The SCC map is available only on FC ports and contains only a single entry. LUN 0 is a VT5900-
SA controller LUN and access to devices behind the VT5900-SA is handled by using SCC logical
unit addressing.
When a host sends a command to the VT5900-SA, the VT5900-SA will select which map to use
based on the port receiving the command and the ID of the host sending the command. For FC
ports the host ID is the World Wide Name and for SCSI buses the host ID is the initiator ID (0 -
15). When a host is unknown to the VT5900-SA or is not attached to a specific map, the VT5900-
SA will use the default setting for mapping. The default setting for each port can be set to Auto-
assigned, Indexed, or SCC (which applies to FC ports only) by the user.
Buffered Tape Writes
Buffered Tape Writes is an option designed to enhance system performance. By returning status
on consecutive write commands prior to the tape device receiving data, Buffered Tape Writes
remove the latency of waiting for responses from the tape device. In the event that data does not
transfer correctly for any reason, the VT5900-SA will return a check condition on a subsequent
command. The VT5900-SA Virtual TapeServer SCSI-to-FC Adapter has a maximum buffer size
of 10.
Commands other than Write(6) are not issued until status is received for any pending write. Also,
status is not returned until the device completes the command. For instance, when a synchronizing
command is sent to a drive, such as sending a Write File mark, a good status means all prior
commands have been successfully completed and data has been successfully written to the
medium. This is appropriate for such tasks as file backup/restore.
For more information, see the SCSI Configuration sections in “Chapter 5, VT5900-SA
Management” and “Chapter 6, Using Visual Manager.”
This completes “Chapter 4, Addressing and Logical Unit Management”.
Note: If the application requires confirmation of individual blocks being written to the medium,
such as audit trail tapes or log tapes, this option should be disabled.