NAME
st_large_recs — determines maximum logical record size allowed through the stape driver
VALUES
Allowed values
0 (off)
or non-zero (on)
Recommended values
0 (off)
or non-zero (on)
DESCRIPTION
This tunable governs the maximum size of records allowed for tape device I/O through
the stape driver.
Larger records can be sent, but the I/O subsystem will split the request into
multiple requests with a maximum record size determined by this tunable.
For instance, if a read of 512K were sent to a tape device on a system with
MAXPHYS
at 256K, two scenarios would exist.
With
st_large_recs
set to any positive integer, the read would be sent requesting a record
size of 512K, and two read requests of record size 256K would be sent with
st_large_recs
set to
0 (off).
Although the stape driver allows a maximum record size of 16MB-1 with
st_large_recs
set to a positive integer, the actual maximum record size supported on a particular
system may be restricted by the maximum I/O size supported by the tape device,
the SCSI subsystem, or interface, for example FC, SCSI, etc.
Who Is Expected to Change This Tunable?
Anyone.
Restrictions on Changing
Changes to this tunable take effect on the next boot.
When Should the Tunable Be Turned On?
When on a system with SCSI subsystem and tape device support for record sizes
greater than
MAXPHYS
(512K), this tunable should be set to
non-zero (on).
What Are the Side Effects of Turning the Tunable On?
If the tape device, SCSI subsystem, or interface do not support larger I/O
record sizes, requests may be split into records that are too large
causing a returned error of
EINVAL.
When Should the Tunable Be Turned Off?
When on a system with SCSI subsystem, tape device, or interface support for
maximum record sizes of
MAXPHYS
(512K), this tunable should be set to
0 (off).
What Are the Side Effects of Turning the Tunable Off?
If the tape device, SCSI subsystem, and interface allow larger records than
MAXPHYS,
you may see slower I/O throughput because the full potential I/O
throughput of the configuration is not being used.
What Other Tunables Should Be Changed at the Same Time?
If on a system that supports large I/O requests like a V-class, you would also
want to be sure the tunable
scsi_maxphys
is larger than 16MB-1.
WARNINGS
All HP-UX kernel tunable parameters are release specific.
This parameter may be removed or have its meaning changed in
future releases of HP-UX.
Installation of optional kernel software, from HP or other vendors,
may cause changes to tunable parameter values. After installation,
some tunable parameters may no longer be at the default or
recommended values. For information about the effects of
installation on tunable values, consult the documentation
for the kernel software being installed.
For information about optional kernel software that was
factory installed on your system, see
HP-UX Release Notes
at
http://docs.hp.com.
AUTHOR
st_large_recs
was developed by HP.
