HP-UX Reference (11i v3 07/02) - 7 Device (Special) Files, 9 General Information, Index (vol 10)

s
scsi_ctl(7) scsi_ctl(7)
ii Two hexadecimal digits, identifying the controlling interface card by its "Instance" number. The
Instance value is displayed in ioscan(1M) output, under column
I for the "Interface" hardware
type.
t One hexadecimal digit identifying the drive (target) address.
l One hexadecimal digit identifying the logical unit number (LUN) within the device.
0 Hexadecimal digit zero, for reserved portion of the minor of the minor number.
o Optional values as follows:
0 To perform Inquiry on open to to ensure the device exists (recommended); or
2 To inhibit Inquiry on open. Starting with HP-UX 11i V3, option 2 is deprecated. It is main-
tained for binary compatibility with existing applications already setting it. Inquiry command
will actually be sent during open, regardless of this option being set or not to 2.
SCSI Communication Parameters
HP-UX supports the SCSI device protocol on parallel SCSI interfaces, Fibre Channel interfaces, and Serial
Attached SCSI interfaces. The SCSI communication parameters described here might only apply to certain
SCSI interfaces and are noted as such in the descriptions.
SCSI communication parameters control features related to communication for three different scope levels:
bus (link), target, and logical unit number (LUN). Bus communication parameters apply to all targets con-
nected to a specific bus. Target communication parameters apply to all LUNs associated with a specific tar-
get. LUN communication parameters apply to a specific LUN. SCSI communication parameters apply to
all device drivers (both device-specific and
scsi_ctl).
At power-up and after being reset, all parallel SCSI devices and hosts communicate using asynchronous
data transfers. Asynchronous data transfers use request (REQ) and acknowledge (ACK) signaling. The
strict ordering of REQ and ACK signaling simplifies the communication protocol but limits I/O perfor-
mance. A SCSI target and host pair may agree to use synchronous data transfers to increase I/O perfor-
mance.
Synchronous data transfers improve I/O performance by lessening the ordering requirements on REQs and
ACKs. By allowing multiple outstanding REQs, signal propagation delays and temporary rate imbalances
are better tolerated. To make use of synchronous data transfers, a SCSI target and host must negotiate to
determine mutually acceptable maximum REQ-ACK-offset and data-transfer rate parameters.
The maximum REQ-ACK-offset parameter indicates the maximum allowable number of outstanding REQs.
The value zero is used to indicate asynchronous data transfer. Other values indicate synchronous data
transfer. The appropriate value is generally dependent on the size of the receive data FIFO. High values
tend to improve data transfer rates. The maximum data-transfer rate parameter indicates the "burst" data
transfer rate (minimum allowable time between successive synchronous data transfers). A SCSI synchro-
nous data transfer request (SDTR) message, used to initiate the negotiation process, is associated with the
processing of a SCSI command.
At power-up and after being reset, all parallel SCSI devices and hosts communicate using eight-bit data
transfers. A SCSI target and host pair may agree to use sixteen-bit (wide) data transfers to increase I/O
performance. To make use of wide data transfers, a SCSI target and host must negotiate to determine a
mutually acceptable data transfer width parameter. A SCSI wide data transfer request (WDTR) message,
used to initiate the negotiation process, is associated with the processing of a SCSI command.
Some SCSI devices are able to simultaneously manage multiple active commands. Such a device has a
command queue that holds commands for processing. Command queuing can improve I/O performance by
reducing the time spent by the device waiting for new commands from the host. Note that command queu-
ing might not improve I/O performance substantially for devices that support "read-ahead" and
"immediate-reporting" (see scsi_disk(7) and scsi_tape(7)). The SCSI device and host use command tags to
correctly manage these multiple simultaneously active commands. At all times when command queuing is
in effect, each active command being handled by a specific LUN has a unique command tag.
SCSI devices indicate their ability to support the special communication features described above in their
SCSI INQUIRY command data. Normally the SCSI INQUIRY command data and negotiation protocols
allow hosts and devices to determine the optimal communication parameters so that I/O performance is
maximized.
The current operating communication parameters may be determined by use of the:
SIOC_GET_LUN_PARMS, PSIOC_GET_TGT_PARMS (recommended) or SIOC_GET_TGT_PARMS (for
backward compatibility), and SIOC_GET_BUS_PARMS ioctls.
HP-UX 11i Version 3: February 2007 − 2 − Hewlett-Packard Company 155