default_disk_ir(5)                                       default_disk_ir(5)
                          Tunable Kernel Parameters



 NAME    [Toc]    [Back]
      default_disk_ir - enable/disable the use of a device's write cache in
      the SCSI subsystem

 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)

 DESCRIPTION    [Toc]    [Back]
      This tunable enables (1) or disables (0) the Immediate Reporting
      behavior of the SCSI subsystem, also known as Write Cache Enable
      (WCE).  With Immediate Reporting enabled, disk drives that have data
      caches return from a write() system call, including raw writes, when
      the data is cached, rather than returning after the data is written to
      the media.  This sometimes improves write performance especially for
      sequential transfers.

      Cached data can be lost if a device power failure or reset occurs
      before the device writes the cached data to media. Because of these
      risks, the recommended value for this parameter on servers is
      Immediate Reporting disabled (0).

      Although not an option to the mount command, this tunable has a
      profound effect upon filesystem and raw disk performance and,
      conversely, data integrity when system resets occur.  This tunable
      also affects delayed-write versus write-through-filesystem behavior.

    Who Is Expected to Change This Tunable?
      Anyone.

    Restrictions on Changing    [Toc]    [Back]
      Changes to this tunable will take effect when the device is first
      opened.

    When Should the Tunable Be Turned On?
      When a third party application vendor recommends it.  For normal use,
      HP strongly recommends you don't enable this tunable.

    What Are the Side Effects of Turning the Tunable On?
      Since enabling this tunable circumvents the protections provided by
      LVM or RAID, there is a strong risk of filesystem corruption and data
      loss in case of a device power failure or reset.



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






 default_disk_ir(5)                                       default_disk_ir(5)
                          Tunable Kernel Parameters



    When Should the Tunable Be Turned Off?
      HP recommends you always disable this tunable.  This is especially
      true if you don't want to take the risk of filesystem corruption and
      data loss in case of a device power failure or reset.

    What Are the Side Effects of Turning the Tunable Off?
      This might reduce the performance of disk write (not read) access
      while eliminating the risk of filesystem corruption and data loss in
      case of a device power failure or reset.

    What Other Tunables Should Be Changed at the Same Time?
      None.

 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]
      default_disk_ir was developed by HP.

 SEE ALSO    [Toc]    [Back]
      write(2), scsi(7).


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