Bug#648367: device-mapper devices such as dm-crypt always have rotational=1; should inherit "rotational" setting from underlying devices

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Nov 14, 2011 at 01:05:29AM +0000, Ben Hutchings wrote:
> On Thu, 2011-11-10 at 11:59 -0800, Josh Triplett wrote:
> > Source: linux-2.6
> > Version: 3.1.0-1~experimental.1
> > Severity: normal
> > 
> > I use dm-crypt on my system, and I have an SSD.  The underlying disk
> > device, sda, has rotational=0:
> > 
> > ~$ cat /sys/block/sda/queue/rotational
> > 0
> > 
> > However, the device-mapper devices have rotational=1:
> > 
> > ~$ head /sys/block/dm-*/queue/rotational
> > ==> /sys/block/dm-0/queue/rotational <==
> > 1
> > 
> > ==> /sys/block/dm-1/queue/rotational <==
> > 1
> > 
> > ==> /sys/block/dm-2/queue/rotational <==
> > 1
> > 
> > The device-mapper devices should inherit the setting for "rotational"
> > from their underlying devices.
> 
> I'm not sure that's true.  Since their queues should feed into the
> queues for the underlying devices, it may be that they shouldn't
> themselves be scheduled as if they are rotating media.

OK, I suppose I don't actually care what rotational shows for a
device-mapper device backed by rotating media.  The case I care about:
when the underlying media has rotational=0, the dm device definitely
shouldn't have rotational=1.

- Josh Triplett