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Re: CHS / LBA uncertainty

From: Andrew Clausen
Subject: Re: CHS / LBA uncertainty
Date: Sat, 26 May 2001 07:34:04 +1000

Thomas Roelz wrote:
> Hello,
> according to the parted documentation Linux/parted cannot safely
> determine if a device is used in CHS or LBA mode. Therefore parted
> usually prints a message like this on startup:
>    Warning: The operating system thinks the geometry on /dev/sda
>    is 1115/255/63. Therefore, cylinder 1024 ends at 8032.499M.
>    You should check that this matches the BIOS geometry before
>    using this program.
> This means that it is dangerous to use parted in a "naive" way
> ignoring this message because a wrongly detected disk geometry
> could have fatal consequences if the disk is altered with this.

The "fatal" consequence is you won't be able to boot Windows.
(If you are using LBA, then this won't happen).  To make
Windows bootable, you need to reinstall it's bootloader
(via sys)

Also, I have seen Windows obliterate itself (i.e. fry the
file system) when the geometry was wrong (CHS mode only)

I don't know when these things are "activated".  Like, FIPS
always seems to work, which is a miracle, because the FATs
don't get resized.  I don't know how Windows decides to
blindly believe what's in the boot sector, and when to
calculate independently (and thus corrupt the file system)

Same goes for partitions... if it isn't aligned to a cylinder
boundary, sometimes it just pretends it is.  Which *can*
lead to windows not believing what it sees in the boot
sector, and calculating other values (thereby corrupting
the file system, as soon as it writes anything)

Parted plays this game really safely.  It calculates the
same values Windows would do, and tries to make everything
aligned nicely, etc.

> So my question is:
> Why are the *fdisk programs able to do their job in a safe way
> without such a warning message (never heard of problems here)?

People generally don't create FAT partitions with fdisk.
Neither do they resize partitions.

Andrew Clausen

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