Reserve
The RESERVE command is used
to
reserve logical units for the use of
the
Initiator.
With
third-party reservation, the logical units may be reserved for another specified SCSI device.
The
RESERVE and RELEASE commands provide the basic mechanism for contention
resolution in multiple-Initiator systems.
Reserve Command Descriptor Block
(COB)
Bit
Byte
7
I
6
I
I)
I
4
I
3
I
2
I
1
I
0
00
Opcode
=
I6H
01
Logical Unit Number
J
3RDPTY
J
Third
Party
Device ID
J
XTNT
= 0
02
Reservation Identification = 00
03-04
(MSB=03)
Extent
List
Length
= 00 (LSB=04)
01)
Control
Byte
3RDPTY
(Third-Party). A 3RDPTY bit set
to
one (1) indicates
that
the reservation is being
made on behalf of another bus device (the third party). When
3RDPTY
is set
to
zero (0),
third-party reservation
is
not requested.
Third
Party
Device
ID.
This field indicates the identity of the third party. The reservation
can only be released by the party
that
made the reservation.
When a third-party reservation is made,
the
mode parameters of
the
reserving Initiator will be
copied
to
the mode parameters of the third party. This will cause Unit Attention
to
the third
party
with a Sense Key of UNIT ATTENTION (6) and an Additional Sense Code of MODE
SELECT PARAMETERS
CHANGED (2AH).
XTNT
(Extent Reservation).
With
the XTNT bit set to zero (0), this command will
request
that
the entire logical unit be reserved for the exclusive use of
the
Initiator until the
reservation is superseded by another valid
RESERVE command froni
the
same
mitiator
that
made the reservation or until released by a RELEASE command from
the
same Initiator, by
a
BUS DEVICE RESET message from any Initiator, or by a "hard" RESET condition. A
logical unit reservation will not be granted if the logical unit is reserved by another Initiator.
It
will be permissible for an Initiator
to
reserve a logical unit
that
is currently reserved by
that
Initiator.
With
XTNT set
to
zero (0), The Reservation Identification and
the
Extent List
Length fields will be ignored.
If
the
logical unit
is
reserved for another Initiator, the target will respond by returning a
RESERVATION CONFLICT status. .
...
Once a reservation is installed, the reserved logical unit is available only
to
the
Initiator
that
issued
the
RESERVE command, or a specified optional
third
party.
If
any other Initiator
attempts
to
perform a command on the reserved logical unit
the
command will
be
rejected
with
RESERVATION CONFLICT status. Exceptions are
the
RELEASE command, which
will be ignored by
the
target, and
the
INQUIRY command, which will be executed.
A-48
SCSI
Command Descriptions
DRAFT
1/18/93 09:58
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