Fujitsu SCANPARTNER 600C, User Manual

Page: 48 / 95

4-20
4.4.1.1 RESERVE UNIT command: COMMAND phase (initiator
Õ
target)
Where a logical unit can be accessed by two or more initiators, there could be interferences with command
sequences, data, etc. This situation can be avoided by issuing the RESERVE UNIT command before
initiating series of operations.
Once a logical unit has properly accepted the RESERVE UNIT command, it will be occupied by the
initiator that issued the RESERVE UNIT command. If the 3rd party reservation option is supported, the
logical unit might be occupied by another SCSI unit - one having an initiator function - which is specified
TPID. In this condition called "reserved;' the logical unit cannot be accessed from any other initiators. The
reserved condition remains effective until one of the following events take place:
1. The reservation is replaced by a new RESERVE COMMAND from the same initiator that has
reserved the logical unit. (issuing another RESERVE UNIT command with the reservation still
effective does not result in an error. The previously established reservation is released as a result of
2,3, or 4 described below.)
2. The RELEASE UNIT command is issued from the same initiator that has reserved the logical unit.
3. The BUS DEVICE RESET message is sent from any initiator.
4. A hardware reset condition is detected.
The condition in effect after 3 or 4 is indicated by a sense key X'6' (UNIT ATTENTION), which is
returned in response to a subsequent command.
When a logical unit is already reserved by another initiator, if a command other than RELEASE
UNIT, INQUIRY, or REQUEST SENSE is issued, the target returns the following status:
Status: B'01100' (RESERVATION CONFLICT)
The initiator having reserved a logical unit can change the reservation by issuing the RESERVE
UNIT command to the same logical unit.
The command descriptor block (CDB) of this command is shown in the following illustration.
7
6 5
4 3 2 1 0
Operation code X'16'
Logical unit number TP TPID
(Reserved)
(Reserved)
Control byte
(a) TP (third party): Byte 1
If the 3rd party reservation option is not supported, setting this bit to 1 causes the target to return
the
following error:
•
Status key: B'00001 '(CIIECK CONDITION)
•
Sense key:
X'5' (ILLEGAL
REQUEST)
This scanner does not support the 3rd party reservation option.
(b) TPID (third party device ID): Byte 1
This scanner ignores TPID.
Byte 0
1
2
3
4
5