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

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.

I do wonder whether rotational should still be the default - almost all
real disks are handled by a small number of drivers like sd, while there
is a wide variety of networked, stacked and virtual disk drivers in use.


Ben Hutchings
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