HP-UX Reference (11i v1 00/12) - 5 Miscellaneous Topics, 7 Device (Special) Files, 9 General Information, Index (vol 9)

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STANDARD Printed by: Nora Chuang [nchuang] STANDARD
/build/1111/BRICK/man7/!!!intro.7
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s
scsi_ctl(7) scsi_ctl(7)
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 com-
mand 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 maxim-
ized. The current operating communication parameters may be determined by use of the:
SIOC_GET_LUN_PARMS, SIOC_GET_TGT_PARMS, and SIOC_GET_BUS_PARMS ioctls.
Occasionally, it is desirable to limit SCSI communication parameters to work around a communication prob-
lem or to provide external insight in determining optimal parameters.
SCSI communication parameter limit
suggestions can be specified by use of the:
SIOC_SET_LUN_LIMITS
, SIOC_SET_TGT_LIMITS
, and
SIOC_SET_BUS_LIMITS
ioctls. Note that there might be substantialdifferences between specified com-
munication parameter limit suggestions and the corresponding actual current communication parameters
being used for communication. These differences are a result of device-specific driver capabilities, interface
driver capabilities, interface hardware capabilities, device capabilities, delays due to the negotiation pro-
cess, delays due to currently active commands, and delays due to commands waiting to be sent to devices.
Note that communication parameter limit suggestions might not survive between
close() and open()
calls, when no SCSI device drivers (device-specific or
scsi_ctl) have associatedLUN(s) open.
The current
SCSI communication parameter limit suggestions may be determined by use of the
SIOC_GET_LUN_LIMITS
, SIOC_GET_TGT_LIMITS, and SIOC_GET_BUS_LIMITS
ioctls.
Logical unit communication parameters may be managed by use of the
SIOC_GET_LUN_PARMS,
SIOC_SET_LUN_LIMITS
, and SIOC_GET_LUN_LIMITS ioctls.
The
SIOC_GET_LUN_PARMS
ioctl indicates the current LUN communication parameter values. The
max_q_depth field indicates whether or not tagged queuing is enabled, and if enabled, the maximum
number of simultaneously active commands allowed. When
max_q_depth is zero, tagged queuing is dis-
abled. When it is one, tags are being used but commands are still being serially processed. When it is
greater than one, tags are being used and
max_q_depth specifies the maximum number of simultane-
ously active commands allowed.
The SIOC_SET_LUN_LIMITS
ioctl may be used to provide LUN communication parameter limit sugges-
tions. The
max_q_depth field indicates whether or not tagged queuing should be enabled, and if
enabled, the maximum number of simultaneously active commands that should be allowed. The
SIOC_GET_LUN_LIMITS ioctl indicates the current LUN communication parameter limit suggestions.
Target communication parameters may be managed by use of the SIOC_GET_TGT_PARMS,
SIOC_SET_TGT_LIMITS, and SIOC_GET_TGT_LIMITS ioctls to any associatedLUN.
The SIOC_GET_TGT_PARMS
ioctl indicates the current target communication parameter values. The
width, reqack_offset, and
xfer_rate fields indicate the currently negotiated data transfer
parameters. When
width is eight, narrow transfers are in effect. When it is sixteen, wide transfers are in
effect. When reqack_offset
is zero, asynchronous transfers are in effect and xfer_rate is meaning-
less. When
reqack_offset is non-zero, synchronous transfers are in effect and the maximum ‘‘burst’’
data transfer rate is
xfer_rate words per second, where the size of a word is as indicated in width.
The SIOC_SET_TGT_LIMITS ioctl specifies the target communication parameter limit suggestions. The
max_width field specifies maximum bus width that should be used for data transfers. The
max_reqack_offset field specifies the maximum number of outstanding REQs that should be
attempted during data transfers. The
max_xfer_rate field specifies the maximum ‘‘burst’’ data rate
that should be allowed during synchronous data transfers. The SIOC_GET_TGT_LIMITS ioctl indicates
the current target communication parameter limit suggestions. The
width, reqack_offset
,
xfer_rate, max_width, max_reqack_offset, max_xfer_rate fields only apply to parallel
SCSI.
Bus communication parameters may be managed by use of the
SIOC_GET_BUS_PARMS,
SIOC_SET_BUS_LIMITS
, and SIOC_GET_BUS_LIMITS ioctls to any associated LUN.
Section 7−−116 − 2 − HP-UX Release 11i: December 2000
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