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Kernel Tunable Parameter
HP-UX 11i Version 3: February 2007

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default_disk_ir — enable and disable use of device's write cache in the SCSI subsystem (OBSOLETE)



0 (off)


0 (off)

Allowed values

0 (off) or non-zero (on)

Recommended values

0 (off)


Note: This tunable is obsolete. HP no longer allows enabling, by default, write cache for all direct access devices on the system. It is still possible to enable write cache for a specific device through the scsictl command. See scsictl(1M).

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?


Restrictions on Changing

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.

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?



All HP-UX kernel tunable parameters are release specific. This parameter has been obsoleted for HP-UX 11i Version 3.

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.


default_disk_ir was developed by HP.


write(2), scsi(7).

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