what is the difference between device-mapper and udev?

[mtonkinson]

I am curious what the difference between device-mapper and udev is. I had a post I made earlier where I thought when I updated to KDE4 something happened that was causing troubles mounting an external and secondary hard drive. I now have determined that it appears to be device-mapper. I downloaded the dmsetup utility a while back and when I type dmsetup ls I get:

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[root@localhost /]# dmsetup ls
1WD_6400AAK_External    (253, 0)
1WD_6400AAK_Externalp1  (253, 1)
SATA_WDC_WD2000JD-22WD-WCAL73024837     (253, 2)

and if I try to mount the drives or access them through Dolphin I get a message that the drives are busy or already mounted. If I enter dmsetup remove_all, this problem disappears. What is the purpose of device-mapper? Is there a way to remove device-mapper or a way to stop device-mapper from mapping my drives? From what it appears, device-mapper is having problems mapping the drives but udev is able to map and mount the drives.

[Bald Brick]

device mapper:
http://linuxgazette.net/114/kapil.html
This may be a bit technical but it's also very informative.

udev:
http://en.wikipedia.org/wiki/Udev

udev creates the device nodes but needs some assistance from
HAL:
http://en.wikipedia.org/wiki/Hardware_abstraction_layer
man hald