Why does the OpenBSD partition start at sector 63?

Why does the OpenBSD partition start at sector 63?

Post by Mark Andrew » Wed, 13 Nov 2002 14:20:54



The behavior of fdisk, when used to define an OpenBSD partition on a
i386 PC, is confusing. During the installation of  the basic
operating system, if you elect to use the entire disk for OpenBSD, fdisk
starts the OpenBSD partition at sector 63 (typically the
second track of the boot disk). However, if you do not use the entire
disk for the OpenBSD partition, fdisk lets you start the
OpenBSD partition at sector 0.

As i found out after an upgrade, starting the partition at sector 0
causes major problems. I've since reinstalled the software, starting
the OpenBSD partition at sector 63.

The documentation explaining why the OpenBSD partition has to start at
sector 63 is sparse at best.

Why does the OpenBSD partition have to start at sector 63? What is
located in sectors 0 thru 62 that should not be overwritten
by the OpenBSD partition? I know it's something to do with the master
boot record (MBR). Is the entire first track of the disk
reserved for it and it's associated data structures? A bit of detail
would be appreciated and the fdisk(8) man page should mention
the need for the OpenBSD partition to start at sector 63 on this platform.

Thanks

 
 
 

Why does the OpenBSD partition start at sector 63?

Post by Christian Loren » Wed, 13 Nov 2002 20:30:36




>>The behavior of fdisk, when used to define an OpenBSD partition on a
>>i386 PC, is confusing. During the installation of  the basic
>>operating system, if you elect to use the entire disk for OpenBSD, fdisk
>>starts the OpenBSD partition at sector 63 (typically the
>>second track of the boot disk). However, if you do not use the entire
>>disk for the OpenBSD partition, fdisk lets you start the
>>OpenBSD partition at sector 0.

>>As i found out after an upgrade, starting the partition at sector 0
>>causes major problems. I've since reinstalled the software, starting
>>the OpenBSD partition at sector 63.

>>The documentation explaining why the OpenBSD partition has to start at
>>sector 63 is sparse at best.

>>Why does the OpenBSD partition have to start at sector 63? What is
>>located in sectors 0 thru 62 that should not be overwritten
>>by the OpenBSD partition? I know it's something to do with the master
>>boot record (MBR). Is the entire first track of the disk
>>reserved for it and it's associated data structures? A bit of detail
>>would be appreciated and the fdisk(8) man page should mention
>>the need for the OpenBSD partition to start at sector 63 on this platform.

>>Thanks

> Yep, that's where the MBR is. I assume the OpenBSD docs don't cover it
> because it's assumed knowledge.

??
I do run OBSD with 0 to xxx without any Problems ... can anyone explain
this?

 
 
 

Why does the OpenBSD partition start at sector 63?

Post by Christian Weisgerb » Wed, 13 Nov 2002 21:21:49



Quote:> The documentation explaining why the OpenBSD partition has to start at
> sector 63 is sparse at best.

It is?  Have you read the accompanying INSTALL.* docs which explain
in rather excruciating detail the mess that BIOS partitioning and
booting are on i386?

Quote:> Why does the OpenBSD partition have to start at sector 63?

Because BIOS partitions start on track boundaries by convention.

Quote:> What is located in sectors 0 thru 62 that should not be overwritten
> by the OpenBSD partition? I know it's something to do with the
> master boot record (MBR). Is the entire first track of the disk
> reserved for it and it's associated data structures?

I see you did not read the documentation.

--

 
 
 

Why does the OpenBSD partition start at sector 63?

Post by Mark Andre » Thu, 14 Nov 2002 05:06:07




> > The documentation explaining why the OpenBSD partition has to start at
> > sector 63 is sparse at best.

> It is?  Have you read the accompanying INSTALL.* docs which explain
> in rather excruciating detail the mess that BIOS partitioning and
> booting are on i386?

Yes, i had read INSTALL.i386 and INSTALL.mbr, but not INSTALL.pt, which
is where the information is located. I didn't think to look at that
text file.

Quote:

> > Why does the OpenBSD partition have to start at sector 63?

> Because BIOS partitions start on track boundaries by convention.

> I see you did not read the documentation.

Well, i read what i thought was the pertinent documentation. Apparently
not. Still, the conventions about partitioning an ATA (IDE) disk should
be at least noted in the fdisk manual page or the installation instructions
for OpenBSD. I'm not the only person who has run into this problem before.

Thanks for the information.

 
 
 

1. fdisk problems with large disks (255 heads, 63 sectors/track) ?

Hi!

I use a RAID Array (Promise FastTrak66) with ca. 60GB. There I have
installed an ext2 Partition, beginning at 3 GB and 3 GB size.

If I use a DOS tool to make a partition (i.e. efdisk oder fdisk), FAT32/NTFS
oder ext2, then the partition ends at head 254 and sector 63. Using Linux
fdisk the partition ends at head 63 und sector 32. All other partitions
(with partition end at head 254 and sector 63) are displayed with "incorrect
end of partition" by Linux fdisk.

Using DOS/Windows I see the reverse situation: PartitionMagic shows the ext2
partition faulty, because it doesnt end at head 254 and sector 63, but head
63 and sector 32 ...

Id appreciate any advice, what could be the problem and how to solve it
(without installing new).

Regards,
Thomas.

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