No Sectors left of 120GB Drive

[R. G. Newbury]

Message: 10 Date: Mon, 2 Oct 2006 15:32:14 -0700 From: "jdow" 
<jdow at earthlink.net> Subject: Re: No Sectors left of 120GB Drive


 >>> >>> At 11:02 AM -0400 10/2/06, Tom Diehl wrote:
 >>> >>>  ...
 >>>> >>> >Will it let you make another primary partition?? I cannot 
remember
 >>>> >>> >if you can have 2 extended partitions or not.
 >>> >>>
 >>> >>> I think one /can/, but it is contrary to specification (which you
 >>> >>> showed below), which says there can be only one Extended partition
 >>> >>> in a Basic partition table.  There can certainly be both an 
Extended
 >>> >>> and an LVM partition in a Basic partition table.
 >> >>
 >> >>but wouldn't that LVM correspond to just a primary partition?  thus
 >> >>not violating the general rule of only one extended partition?
 > >
 > > Yes.

 >In an abstract sense I'd love to know what limits a disk to only one
 >extended partition other than legacy and sloppy code. I've had two
 >extended partitions on other systems before. I think I did that with
 >the 2.0.x kernel tree at one time, too. All that's needed is simply
 >traversing the partition tables sensibly.


Having had to rebuild partition tables a few times from some 
over-enthusiastic misuse of Partition Magic, the answer is that:

a)	partition table space is limited to four primary partition entries 
(this is 'legacy specification');
b)	any of those entries can be a primary partition of an extended 
partition type, but you cannot have any primary partitions after that entry;
c)	a logical partition entry in an extended partition has a pointer to 
the start of next logical partition: the system follows the chain to 
find subsequent partitions;
d)	normally, the partition chain is linearly increasing, but you can 
have entries which jump 'back'. This completely defeats some operating 
systems, especially OS/2 (IIRC Windows does not mind this: I don't know 
about Linux and I have NO desire to find out the hard way!); and
e)	you may have had 2 extended partitions by creating an extended 
partition INSIDE the first extended partition. If you use a program 
which lets you change the table entries directly (partedit), this would 
be easy to do, since you are only changing a type byte, but I wouldn't 
do that unless I was testing to see if the OS could handle it, meaning, 
the drive had no important information on it, which naturally, was never 
the case, when I had to figure out what had gone wrong with my drives!

Partition Magic creates partitions in monotonic order notwithstanding 
that you may be jumping 'back' to fill a gap. It does NOT go back and 
change the pointers to push the new partition into the chain. If you are 
careful to move and expand partitions starting from the bottom and only 
create partitions in free space at the end, you can avoid this 
problem...maybe!
I have no idea if the linux-based equivalent tools work the same way, 
but I strongly suspect they do due to the very nature of the beast. 
Backup the table first and backup your data.


              R. Geoffrey Newbury			
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