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Re: Strange MBR CHS partition values created by debian-installer



Hi Valdikss,

On Sat, Jul 08, 2023 at 09:17:54PM +0300, ValdikSS wrote:
>Hello list,
>
>I was getting very slow boot speeds on old WYSE C10LE thin client with VIA C7
>CPU and Phoenix bios (year 2008). This is due to combination of BIOS, GRUB,
>and what I assume Debian-installer bugs.
>
>Long story short, Debian writes strange/incorrect C/H/S values to the MBR
>partition table upon installation, to which testdisk software complains as
>"Bad relative sector".

Nothing should be caring about C/H/S at all in the 21st century. Using
C/H/S only allows you to access 515MB of disk [1]. *Everything* these
days uses LBA instead.

What makes you think that the BIOS on this old machine cares about
C/H/S? What happens if you tweak the partitioning by hand when
installing?

>I've tried to install Debian 12 i386 to a 8G disk, using qemu, with guided
>automatic partitioning. Testdisk data right after the installation:
>
>> Disk testz.img - 8589 MB / 8192 MiB - CHS 1045 255 63
>> Current partition structure:
>>      Partition                  Start        End    Size in sectors
>> 
>>  1 P Linux                    0  32 33   919 199 48   14774272
>> 
>> Bad relative sector.
>>  2 E extended               919 232 16  1044  52 32    1996802
>> 
>> Bad relative sector.
>> No partition is bootable
>>  5 L Linux Swap             919 232 18  1044  52 32    1996800
>> 
>> Bad relative sector.
>
>
>It seems that testdisk automatically recalculates C/H/S values and shows
>corrected data (in the table above).
>
>Here's what really is present in the MBR (data of the first partition entry):
>
>> $ ./mbr_my.py testz.img Status:                  0x0
>> C/H/S start:             4 4 1
>> Part type:               0x83
>> C/H/S end:               1023 254 2
>> LBA of first sector:     2048
>> Sector count:            14774272
>
>
>fdisk/cfdisk and parted all create partitions for which testdisk does not
>complain.

d-i drives parted to make partitions...

[1] https://forums.tomshardware.com/threads/lba-chs-bios-prob.685748/


-- 
Steve McIntyre, Cambridge, UK.                                steve@einval.com
"We're the technical experts.  We were hired so that management could
 ignore our recommendations and tell us how to do our jobs."  -- Mike Andrews


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