What is the significance of /queue/rotational in Linux?

Question

I was searching to identify the way to detect whether a disk is SSD or HDD? I found that there is a way to detect it. This is by reading the value of cat /sys/block/sda/queue/rotational? If it is 1 then it is HDD otherwise it is SSD. I was wondering what is this file /sys/block/sda/queue/rotational? Why is it used by kernel? What is this k-object that is maintained by kernel?

Answer 1

From: https://www.kernel.org/doc/Documentation/block/queue-sysfs.txt

rotational (RW)

This file is used to stat if the device is of rotational type or non-rotational type.

See also: http://lwn.net/Articles/408428/

Traditionally, the block layer has been driven by the need to avoid head seeks; the use of quite a bit of CPU time could be justified if it managed to avoid a single seek. SSDs - at least the good ones - care a lot less about seeks, so expending a bunch of CPU time to avoid them no longer makes sense. There are various ways of detecting SSDs in the hardware, but they don't always work, especially with the lower-quality devices. So the block layer exports a flag under

/sys/block/<device>/queue/rotational

which can be used to override the system's notion of what kind of storage device it is dealing with.