On Thu, Jun 25 2009, Linus Torvalds wrote:

On Thu, 25 Jun 2009, Martin K. Petersen wrote:

quoted

Neil> So I asked git why it as added, and it pointed to
Neil>   commit 1308835ffffe6d61ad1f48c5c381c9cc47f683ec

Neil> which suggests that it was added so that user space could tell the
Neil> kernel whether the device was rotational, rather than the other
Neil> way around.

There's an option to do it via udev for broken devices that don't report
it.  But both SCSI and ATA have a setting that gets queried and the
queue flag set accordingly.

.. except few devices actually set it. 

That flag is _definitely_ all about the user being able to override it.

Most certainly, the idea was to add udev rules to set it for drives.
Fortunately newer drives to work right without a need for such rules,
but it should be handy for the ones released last year and earlier.

Most user space will not care what the setting is, it's mostly for
internal use. CFQ uses it, as does btrfs to decide allocation policy.

-- 
Jens Axboe