multi-OSes and partitioning

[taeb]

Hi:

I've been using Linux for some time on an old Mac, but recently started
using it on a PC, and discovered the four-partition limit.  It has taken
me a while to get my foggy brain wrapped around how that works -- and
I'm still wrapping.  I'd like to get some feedback on this topic.

Even though I don't run any M$ software on my machine, it still has the
4-partition limit -- well, top-level limit.  That's required because of
the BIOS, right?

At first I thought this limit meant only four systems per disk, but
some of the stuff I've read says you can have an arbitrary number of
sub-partitions in an extended partition.  OK, so one primary (required I
think) and then the rest of the disk an extended partition which I can
sub-divide into an arbitrary number of partitions to hold other OSes.
Easy --  except the non-linux OSes I've read about say they need a
primary partition.  Why?  There must be some distinction between primary
and extended other than the ability to sub-divide the extended.

One of the man pages -- fdisk I think -- said you can have swap as a
(primary) partition or a sub-partition, but the primary partition was
more efficient.  What is the extra overhead involved in accessing
sub-partitions, and doesn't that imply a performance hit for accessing,
say, /usr in a sub-partition rather than a primary?  How much of a hit
is it?

Also, if I have swap in a primary partition I presume I can use the same
swap area for different OSes.  Couldn't I also share the swap if it were
in a sub-partition?

Finally, the pertinent info on my PC system is:  P4-630, 2GB RAM, 1 SATA
DVD-RW and 1 SATA 160GB HDD.  Although Ubuntu is my primary system I want
to have three other OSes also: freeDOS, because I might need to update the
BIOS some day; Minix3 -- if it ever supports my SATA DVD-RW -- because I
think it's interesting; and freeBSD to see what it's like.

The partitioning scheme I'm planning to use is:

P1: freeDOS     ~100MB
P2: minix3      ~900MB
E3: freeBSD     ~59GB
E4: Linux       <remainder of disk>

Inside E4 would be partitions 5-9 laid out something like:
p5: Linux-1     to hold one version of Ubuntu -- current or next
p6: Linux-2     to hold another version of Ubuntu
p7: swap
p8: Home


Any comments on conflicts or problems or whatever?


My thanks for any replies, and my apologies for such a lengthy posting.

Regards,
tonyB