Re: Disk mapping confusion with the device command

[Molle Bestefich]

Phillip Susi wrote:
> device (hd0,4) /dev/mapper/via_hfciifae5
> device (hd0) /dev/mapper/via_hfciifae

AFAIR, there's a nasty bug in Grub regarding the ordering of these commands.
Doing the above in opposite order may give you radically different results.

I ran into the same problem when I installed on my fakeraid.
Considered fixing it, but then didn't :-).

> When I issue the root (hd0,4) command I get an error message stating
> that the partition table is corrupt.  When I enable debug messages and
> verbose mode, I see a debug message saying that it is reading from
> /dev/mapper/via_hfciifae5.  It looks like grub is looking for the MBR on
> the partition block device instead of the disk block device, and I think
> that is a bug.

Yeah, pretty sure I saw that too.. hehe...

> I left out that line and then had to use the geometry command to specify
> the correct geometry for the drive and now it's working.

Oh, there's a bug I actually fixed.  Easy too.
Problem is that HDIO_GETGEO won't work for device-mapper devices.

My patch could use a bit of TLC, for example it would be nice if it
worked with CHS mode too:
 * modprobe edd
 * match the device to /sys/firmware/edd/int13_??? and find BIOS idea
of correct nr. of heads (instead of defaulting to 255)

There's a couple of other nits too.
Doesn't seem like my fix is going to get into Grub anyway, so I
stopped working on it, heh.

> I'm now using grub to dual boot windows and ubuntu on a sata hardware
> fakeraid raid 0.  If anyone is interested in how, instructions can be
> found here:
>
> https://wiki.ubuntu.com/FakeRaidHowto

Thanks for sharing!