st_large_recs(5)                                           st_large_recs(5)
                          Tunable Kernel Parameters



 NAME    [Toc]    [Back]
      st_large_recs - determines maximum logical record size allowed through
      the stape driver

 VALUES    [Toc]    [Back]
    Failsafe
      0 (off)

    Default    [Toc]    [Back]
      0 (off)

    Allowed values    [Toc]    [Back]
      0 (off) or non-zero (on)

    Recommended values    [Toc]    [Back]
      0 (off) or non-zero (on)

 DESCRIPTION    [Toc]    [Back]
      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    [Toc]    [Back]
      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].




 Hewlett-Packard Company            - 1 -      HP-UX 11i Version 2: Sep 2004






 st_large_recs(5)                                           st_large_recs(5)
                          Tunable Kernel Parameters



    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    [Toc]    [Back]
      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    [Toc]    [Back]
      st_large_recs was developed by HP.

 SEE ALSO    [Toc]    [Back]
      scsi_maxphys(5), scsi_tape(7).


 Hewlett-Packard Company            - 2 -      HP-UX 11i Version 2: Sep 2004