Queued I/O Processes; Untagged Queuing; Tagged Queuing - Seagate ST11200 Product Manual

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4.7

Queued I/O processes

Queuing of I/O processes allows a disc drive to accept multiple commands for execution at a later time.
There are two methods for implementation of queuing, tagged and untagged. Tagged queuing allows the disc
drive to accept multiple commands from each initiator. Untagged queuing allows the disc drive to accept one
command from each initiator. Drives that have SCSI-2 implementation support tagged queuing while in SCSI-
1 or SCSI-2 mode. They can use untagged queuing mode if the initiator does not send queue tag messages.
Initiators may add or delete commands to the queue for the a disc drive within the limitations specified in this
specification. When adding a command, the initiator may specify fixed order of execution, allow the disc drive
to define the order of execution, or specify that the command is to be executed next. See glossary in Section
2.0 for terminology definitions when reading the following explanations.
4.7.1

Untagged queuing

Untagged queuing allows the disc drive to accept a command from an initiator while a command from an-
other initiator is being executed. Only one command for each I T L nexus may be accepted at a time.
A new I/O process may be initiated any time the BUS FREE phase exists even if another I/O process from a
different initiator is being executed. If the disconnect privilege is not granted, the disc drive returns BUSY
status to the new I/O process.
The I T L nexus specifies the relationship so that the disc drive can always reconnect to the initiator to restore
the pointers for I/O process as long as only one command per I T L nexus is issued. It is the responsibility
of the initiator to assure that only one command is issued at any time.
4.7.2

Tagged queuing

Tagged queuing allows a disc drive to accept multiple commands from the same or different initiators until the
disc drive's I/O process queue is full. A new I/O process may be initiated any time the BUS FREE phase
exists, if the disconnect privilege is granted. If the disconnect privilege is not granted for a tagged command
the disc drive returns BUSY status to the new I/O process.
The queue tag messages (see Table 3.5.3-2) allow the initiator to establish a unique I T L Q nexus to identify
each I/O process. Each I/O process may be a command or a set of linked commands with a unique queue
tag.
The I T L Q nexus allows the target to reconnect to the desired I/O process and the initiator to restore the
correct set of pointers. An initiator may have several I/O processes ongoing to the same or different logical
unit as long as each has a unique nexus.
If only SIMPLE QUEUE TAG messages are used, the disc drive may execute the commands in any order
that is deemed desirable within the constraints of the queue management algorithm specified in the control
mode page (see Table 5.2.1-28). The command ordering is done by the disc drive to meet its performance
and functional goals. The algorithm used by the disc drive attempts to achieve certain drive or system perfor-
mance goals established in the drive firmware for the queued commands and guarantee that all commands
will be executed. One possible goal would be to minimize seek times, but there could be others, possibly
designed to meet some special system need. Commands from other initiators are also executed in an order
selected in the same manner. The disc drive uses the simple Queue Tag when reconnecting to the initiator.
Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E