The RESERVE and RELEASE commands define how different types of restricted access may be achieved, and to whom the access is restricted. This subclause describes the interaction of the initiator that requested the reservation, and the other initiators.
An initiator uses reservations to gain a level of exclusivity in access to all or part of the medium for itself or another initiator. It is expected that the reservation will be retained until released. The direct-access device must ensure that the initiator with the reservation is able to access the reserved media within the operating parameters established by that initiator.
The following paragraphs explain, on a command by command basis, the appropriate target response when a reservation exists. Unless otherwise noted, the appropriate response to an initiator that issues a command to a direct-access device that is reserved to another initiator is RESERVATION CONFLICT status.
The CHANGE DEFINITION command is dealt with as follows. If any initiator has an extent reservation on a direct-access device, no other initiator may affect the operating definition of that initiator by use of this command. If the direct-access device allows different operating definitions for each initiator, then there is no conflict; otherwise, a reservation conflict occurs.
The COMPARE, COPY, and COPY AND VERIFY commands are evaluated for reservation conflict as if they were normal write and read operations even when a direct- access device is requested to copy to or from itself. For example, if a COPY is issued to logical unit 0 that requests the direct-access device to copy from logical unit 0 to logical unit 1, access to logical unit 1 must also be evaluated for conflict.
The FORMAT UNIT, PREVENT ALLOW MEDIUM REMOVAL (with a prevent bit of one), REZERO UNIT, and START STOP UNIT commands return a RESERVATION CONFLICT status if any other initiator has an extent reservation on a direct-access device.
The INQUIRY and REQUEST SENSE commands are not affected by any kind of reservation.
The LOG SELECT, LOG SENSE, MODE SENSE, TEST UNIT READY, READ CAPACITY (PMI set to zero), READ BUFFER, WRITE BUFFER, and READ DEFECT DATA commands are not affected by extent reservations.
The SEEK, LOCK UNLOCK CACHE, PRE-FETCH, and SYNCHRONIZE CACHE commands are evaluated for reservation conflict as if they were normal write or read operations.
The MODE SELECT command is dealt with as follows. If an initiator has an extent reservation on a direct-access device, and another initiator attempts one of these commands, a reservation conflict occurs if the command affects the manner in which access of the extent by the first initiator is performed. If the command does not affect access to the extent, or parameters are saved for each initiator, then a conflict does not occur.
The SEND DIAGNOSTIC, RECEIVE DIAGNOSTIC RESULTS commands conflict with an extent reservation only if they affect access to the extent (as with MODE SELECT).
The REASSIGN BLOCKS command may not reassign a block that is in an extent reserved to another initiator.
The SET LIMITS command generates a reservation conflict if the logical blocks specified are within an extent reserved to another initiator.
ALL other commands are that request read or write operations are evaluated for reservation conflict as described in the RESERVE command.
When a system is integrated with more than one initiator, there must be agreement between the initiators as to how media is reserved and released during operations, otherwise, an initiator may be locked out of access to a target in the middle of an operation. For example, initiator 'A' has a write operation in progress to a direct-access device which has data stored in cache memory. Then, initiator 'B' issues a RESERVE command to the direct- access device. As a result, initiator 'A' is locked out of issuing a SYNCHRONIZE CACHE command to ensure the integrity of the data. To prevent this from happening, initiator 'A' should issue a RESERVE prior to the write command.