Re: recover partition table

[Stephen Powell <zlinuxman@wowway.com>]

On Sat, 26 Mar 2011 12:36:08 -0400 (EDT), Geronimo wrote:
> 
> I'm a bit confused.
> 
> I tried to read partition info from hex-dump of first block and compare that 
> with the values from /var/log/installer/partman ...
> 
> According to partition info from wiki the biggest number of chs is 0x3FF, 
> which is 1023 decimal - and partman output contains entries like:
> 	(91201,0,0)	(91201,80,62)
> 
> So is there another way to interpret entries from partition table?

The master boot record is the first sector of the disk (512 bytes).
Its LBA value is 0 and its CHS value is 0:0:1 (cylinder 0, head 0, record 1).
The partition table is contained within the master boot record.
Assuming that your disk is partitioned using the standard MS-DOS
disk partitioning scheme, the partition table is at offset 0x1be
(decimal 446).  The total size of the partition table is 64 bytes.
It consists of four 16-byte entries.

Each entry describes a primary partition or an extended partition.
There can be a maximum of one (minimum of zero) extended partitions.
Logical partitions, if any, are described in the extended partition
table in the first sector of the extended partition.  See, for example,

   http://en.wikipedia.org/wiki/Master_boot_record

for more details.

Each partition table entry contains the starting sector of the
partition in both CHS format and LBA format.  It also contains the
total number of sectors in the partition and the CHS address of
the last sector in the partition.  LBA values and CHS values can
be converted back and forth to each other as long as (1) the
disk geometry is known, and (2) the partition is completely
contained within the portion of the disk which is addressable
in CHS format.  In the best case scenario, CHS values can address
only the first 8.4G of the disk.  The LBA values can address up
to 2T (2 terabytes).

Unfortunately, not all disk partitioning programs agree on the
disk geometry; so what to use for LBA values depends on which
program was used to partition the disk.  I address this topic
to some extent in my LILO web page

   http://users.wowway.com/~zlinuxman/lilo.htm

That's why I recommend that you concentrate on the sector numbers,
which are essentially the same as LBA values.  The sector information
is further down in the file than the CHS information.

-- 
  .''`.     Stephen Powell    
 : :'  :
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   `-