*BSD News Article 97220


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From: j@uriah.heep.sax.de (J Wunsch)
Newsgroups: comp.unix.bsd.freebsd.misc
Subject: Re: Sharing swap between FreeBSD and Linux, not enough partitions
Date: 5 Jun 1997 21:34:46 GMT
Organization: Private BSD site, Dresden
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Xref: euryale.cc.adfa.oz.au comp.unix.bsd.freebsd.misc:42498

crs@quail.swcp.com (Charlie Sorsby) wrote:

(partitions vs. slices)

> Or, one could say that SVR4's usage is somewhat confusing since BSD
> predates SVR4...   :)

Not really.  The term `partition' was a long-standing fact in Unix's
tradition, of course.  Up to SVR3.2, there wasn't much confusion about
it, but you were usually also limited to a single fdisk entry for PC
UNIX as well (with some vendor hacks to allow accessing the DOS
portion of the disk).  UNIX had partitions of its own.

With SVR4, the PC UNIX variants apparently decided to throw away their
old meaning of `partition', and take over this term for the fdisk
table units.  To differentiate, their sub-devices were called slices
then.

386BSD 0.0 stood in the tradition of Unix, and didn't knew anything
about fu***nny fdisk tables.  It's got its partitions, of course.

386BSD 0.1 through FreeBSD 2.0 (i've lost the track on the other BSDs
here) came with a hack that allowed sharing the disk with other
systems using the fdisk table, but only allowing for one fdisk unit
with BSD.  Partitions were still the same.  (The hack was to create
another `magic' pseudo-partition, `d', covering the entire disk, while
`c' covered the BSD portion.)  This is comparable to SVR3.2 on PCs.

With the advent of the slice code in FreeBSD 2.0.5, it's now been
possible to handle the fdisk table appropriately.  Bruce Evans, the
architect of the slice code, decided to keep the term `partition' as
it used to be, thus needed another term for the fdisk `super-
partitions', and called them slices.  It would perhaps have been
better to choose a name that doesn't conflict with SVR4's usage, but
that's too late now either.


So, while BSD predates SVR4 (in principle), BSD != BSD, and the
current usage of slices vs. partitions in FreeBSD has really been
introduced after SVR4's precedent case.

FreeBSD can still handle the sliceless case, standing in the Unix
tradition.  It owns the entire disk.  Somehow, the term ``dangerously
dedicated mode'' has been created to describe this kind of setup.  The
best you get by it is that you're largely independent of whatever
`translation' your BIOS might use today.  The only requirement for it
to work is that this translation involves at least 15 sectors per
track (the size of the primary and secondary bootstrap, which must be
loaded initially using the BIOS).  You can easily shuffle a disk
around from an Adaptec to a NCR controller (or swap controllers, as i
did recently), without ever thinking about the BIOS braindeadness
again...  Too bad, some BIOS vendors try to be `smart' these days, and
don't boot a disk if they believe its master boot record contains an
invalid fdisk table. :-(((  HP's PCs are among this, even their more
serious Netservers now.  They should be shot for this braindamage.
(It won't help a winluser with a broken disk very much, either.)

-- 
cheers, J"org

joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/ -- NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)