HP-UX Reference (11i v2 04/09) - 5 Miscellaneous Topics (vol 9)

s
st_large_recs(5) st_large_recs(5)
(Tunable Kernel Parameters)
NAME
st_large_recs - determines maximum logical record size allowed through the stape driver
VALUES
Failsafe
0 (off)
Default
0 (off)
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 posi-
tive 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 parame-
ter 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
Section 5−−338 Hewlett-Packard Company − 1 − HP-UX 11i Version 2: September 2004