Rotational hard disks have a controversial (mis)feature which automatically unloads the read/write heads during per- ceived inactivity, usually within 10 seconds. Western Digital calls it "Intellipark", but the problem is present in other manufacturers' disks as well, and goes back to at least the mid-2000s.

Frequent parking leads to noticable latency on wakeup, and the hard retraction is seen as an impediment to disk life. Even with various utility programs, this parking feature is not reliably deactivatable. (See "Comparison" section below.)

The "parkverbot" daemon will issue small read requests periodically to a random location on disk in an effort to reset the inactivity timer in the hardware and so prevent the dreaded head unloading. As such, its operation is non-destructive and interoperable across different brands of hard drives and transports.

The current block reading algorithm works reasonably well in practice: the worst observed head parking rate was about 3 unloads/day (0.125/hour); best figures hover around 0.5 unloads/day (0.021/hour).


Comparison with other utilities

The "wdidle3" proprietary utility uses a vendor-specific command (VSC) to change the — infamous - "idle3" timer which controls the parking time. How well this works with non-WD drives is unknown; if the VSC modifies firmware, it is probably a very bad idea. The program also requires MS-DOS, which is increasingly hard to run on contemporary x86-PC systems, and totally inacceptable on non-x86/non-PC systems. Its effectiveness from within a virtual machine or emulator using disk/block device passthrough is not known either.

The `hdparm -B` Linux command may be used to set the APM (Advanced Power Management) level. However, disks often report to not support this setting, or if they do, have no effect on unloading, but only acoustic settings. hdparm also does not work with SCSI disks, SAS disks, disks in enclosures attached through USB translation layer, or SATA disks on a SAS plane. (There are dumps for developers in the source distribution.)

"idle3-tools" is something that I only found recently (2014), though it seems to exist for slightly longer than parkverbot. Like wdidle3, it uses the VSC.

"wdantiparkd" WD anti-intellipark daemon says it uses a write operation on a file. That is somewhat unfortunate, as layers of caching in both the operating system and disk may lead to an inactivity beyond the parking threshold on the actual head.

parkverbot — this daemon — does normal read requests on the raw block device. This means there will be no destruc- tive operations, no additional degradation of the medium, and allows it to be usable with all kinds of rotating hard disks no matter the manufacturer or interface type. Chunks are read from random new locations far enough from the old one in an attempt to evade caches. This will manifest in a short flash of the disk LED (if you have one) every now and then. By default, one chunk is requested every four seconds, so that two are within 8 seconds (the common WD idle3 time).