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[dm-devel] Re: Support HDIO_GETGEO on device-mapper volumes

Molle Bestefich wrote:


That doesn't stop the Linux kernel and device-mapper from presenting
usable numbers to user space tools.

What are "usable numbers"?
The only usable numbers are the exact ones that the BIOS uses, because
they are the only CHS numbers that have a purpose:

The purpose of those numbers is backwards compatibility with DOS and
Windows.  Stuff like PartitionMagic, FreeDOS, odd boot loaders
(including Windows XP's), weird implementations of fdisk, etc.

And that's the reason that the kernel and device mapper should either
represent the numbers that the BIOS currently uses, or nothing at all.


Exactly! If guesses have to be made because the correct values can not be obtained from the bios, then it should be fdisk/parted/whatever that has to make that guess ( or ask the user for manual intervention ), not the kernel.

The statement that "any value is gibberish" is plain wrong, since the
BIOS translations are useful values under some circumstances.

In certain dm configurations ( specifically, when used for dmraid assisted hardware fakeraid ) it might make sense, but for many dm configurations it won't because the bios can't access the volume anyhow, so it hardly makes any sense to associate a bios geometry with it. That's what I meant by gibberish; if the drive isn't seen by the bios, any geometry you come up with has no meaning.

In cases where it does make sense, you want a consistent and well
maintained method of matching BIOS drives to devices and reporting
BIOS numbers.

Right. Unfortunately, it seems this was specifically removed from the kernel because it was problematic, which I don't really mind since CHS addressing needs to die anyhow.

In cases where it does not make sense, you just want to return error
and let the app handle it.


Just bring the apps still using it ( grub, lilo ) into the 21st century and have them
stop using these meaningless values in the first place.

LILO is already fit for fight if you use the right flag.

Good idea to fix GRUB.
The GRUB code does look like a mess to me, though.
An ETA on GRUB2 would be nice, or an alternate source tree where
people could improve on the existing GRUB..

IIRC, grub just uses the geometry it finds in the MBR, or you can override with the geometry command.

LBA has been around for a good 10 years now, so I think it is safe to no longer
require these made up values to support CHS addressing.

If you rip all CHS out of the kernel right now, user space tools has
to make guesses when mucking with partition tables.  Any attempts to
make Linux <whatever>fdisk compatible with other fdisk's will be
futile.  Stuff like for example placing a partition on a (virtual)
cylinder boundary is going to break when Linux <whatever>fdisk
translates LBA to CHS in one way (since the kernel doesn't provide a
meaningful value) and your BIOS does it another.

Right, but this has already been done. The values currently in the kernel are not taken from the bios, so as you said, either report it right, or don't report it at all.

In the case where fdisk must initialize a brand new MBR, then it should assume sensible defaults and allow you to override. Most of the time though, people will already have a drive formatted by windows which will have the correct geometry written in the MBR, so the linux tools will just use that.

Ok, the better approach might be to destroy HDIO_GETGEO entirely and
tell userspace apps to use /sys/firmware/edd/int13_dev instead.  It
provides for a cleaner implementation.

But then again, that breaks compatibility with current tools.

They are already broken since the HDIO_GETGEO isn't returning the values from the bios. What is the status of edd? Is that officially supported or is it depreciated? If it is ok to use, then it would probably be a good place for fdisk to get that info from, though it's going to have to figure out which int13_dev ( if any ) corresponds to the device you are partitioning. Sensible defaults for well known devices like /dev/hda should cover most cases, but not all. For instance, I recently configured a server's bios to disable the primary ide channel because that disk was bad and I needed to boot from the second mirrored drive, so /dev/hdb actually corresponded to bios device 80, not /dev/hda.

A good way to get the ball rolling could be if we could all agree that
there are a number of tools that need the BIOS CHS numbers.  Then we
can decide where the universal logic that attempts to guess which
devices are which should go.  Then a proper implementation could be
discussed and conjured, and dmraid, <whatever>fdisk, perhaps LILO/GRUB
etc. could be made dependant on it.  And THEN the HDIO_GETGEO could be

Aye, well put. So let's see here. As far as I can see, the only tools that need this information are ones that can create MBRs, so that's fdisk, cfdisk, parted, and friends. Everything else can/should just use the values listed in the MBR.


Anyway, just a personal idea, your concept of completely nuking CHS
from the kernel right now might be acceptable as well.

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