fdisk head inconsistency

Michael Conrad 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.
(snip)
> 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.

https://en.wikipedia.org/wiki/Master_boot_record#Partition_table_entries

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 ;-)

-Mike

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.busybox.net/pipermail/busybox/attachments/20190930/18b6f0b7/attachment.html>


More information about the busybox mailing list