fdisk head inconsistency
mconrad at intellitree.com
Mon Sep 30 22:30:27 UTC 2019
On 9/29/2019 8:57 AM, Mauro Condarelli wrote:
> Note: the SD card was originally formatted using Debian GNU/Linux fdisk
> on a desktop.
> I have several problems here, possibly all stemming from a bogus
> detection of "disk geometry":
> - (n)ew partition always suggested sector 16 as starting point
> (obviously useless).
> - (p)rint shows bogus values in CHS of newly created Partitions.
> - (v)erify complains about "old" partition geometry.
> Command (m for help): p
> Disk /dev/mmcblk0: 7580 MB, 7948206080 bytes, 15523840 sectors
> 242560 cylinders, 4 heads, 16 sectors/track
> Units: sectors of 1 * 512 = 512 bytes
> Device Boot StartCHS EndCHS StartLBA EndLBA
> Sectors Size Id Type
> /dev/mmcblk0p1 0,33,3 17,96,10 2048 264191
> 262144 128M c Win95 FAT32 (LBA)
> /dev/mmcblk0p2 17,96,11 293,126,14 264192 4458495
> 4194304 2048M 83 Linux
Since you have only run "delete" commands prior to this print command,
I'm assuming that the debian fdisk wrote out those partitions where the
head number (i.e. 33) is exceeding the maximum (4). (but maybe you
could check?) I would guess that maybe the USB stick came with the
partition table listing 4 heads 16 sectors as a hint to partition tools
that the media likes 32K boundaries (4*16*512=32768). Maybe Debian
fdisk ignored those and wrote out CHS offsets based on assumption that
the partition table matches the default geometry that fdisk generates?
Either way, I'm guessing the inconsistency is already written to disk,
so you can fix it (or not) however you like.
In the grand scheme of things, no modern software cares about those CHS
numbers anyway. Each partition table entry contains redundant CHS
start/end and LBA (plain old integer sector number) start/end, and every
modern tool/OS uses the LBA number and ignores the CHS, because the CHS
haven't been real measurements for 25 years or more.
The only thing you should worry about is that the partition is aligned
on a multiple of the erase-size of the USB media. I'm not current on
what those are, but 1MiB should be sufficient, which is why the default
start sector for fdisk is 2048 (*512=1MiB)
I'd just go ahead and try it.... assuming you have all the data on there
backed up, which you should any time you play with partition tables ;-)
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