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From: jsdy@hadron.UUCP (Joseph S. D. Yao)
Newsgroups: net.unix-wizards
Subject: Re: hp.c (4.1bsd)
Message-ID: <100@hadron.UUCP>
Date: Thu, 28-Nov-85 00:29:23 EST
Article-I.D.: hadron.100
Posted: Thu Nov 28 00:29:23 1985
Date-Received: Fri, 29-Nov-85 10:40:06 EST
References: <46@brl-tgr.ARPA>
Reply-To: jsdy@hadron.UUCP (Joseph S. D. Yao)
Organization: Hadron, Inc., Fairfax, VA
Lines: 82
Keywords: partitions, root, swap
Summary: There are standards.

In article <46@brl-tgr.ARPA> expert@lbl-csam.arpa writes:
>  ...  There seem to be some magic numbers in the "size" structure
>(partitioning info.) for the drives, which I don't understand.

Daddr_t is whatever type best holds a disk block address.  This
must be at least 30 bytes.  It should be a long int, but Berkelians
tend to think that if a 16-bit machine falls in the woods, no-one
will hear it.	;-)

You are obviously using a 4BSD system.  I will try to remember the
correct info for both BSD and USG.

For BSD, the root partition is always 15884 sectors ("blocks" is
not a good term here, although the code uses it).  This is nice
and reasonable for an RP0[456].  It is minimal for the others.
Being LCD, it is thus the standard.  This will usually not completely
cover a cylinder on other drives, leading to problems when dd'ing
using a multiple of the cylinder size.  Partition B, often used
as swap, is 33440 sectors long.  Partition C is the whole disk.
Partition H is the next 291346 (?) sectors, if they are there.
Note that, although 4.1 originally set this up to fit exactly
onto an RP06, because 4.2 skips the bad-block tables at the end
of most HP (DB) drives, this no longer fits an RP06!  Anyway,
whether or not partition H is taken out, partition G is the
rest of the disk.  Partitions D, E, and F cover partition G.
Partition D is always 15884 sectors, just like partition A.
Partition E is next, often (not always) 55936 sectors.  And
Partition F is the rest of Partition G.  This allows you to
EITHER use a large partition (G) or three smaller ones (DEF);
OR even to use the whole disc as one FS (C).

Of course, some sites will munge with the tables and break
this standard.  And under Ultrix, this can be changed dynamically
and written on the disk -- something that a comment in the driver
has for a long time said should be done.  I like.  Some of the
munged non-support, I don't like ... in particular, if mkfs
can't ioctl-read a partition table, it will write a zero-length
partition table onto the drive AND MARK IT VALID -- and then
NOTHING can open and access that drive, even to change the
partition table!  (*sigh*)

Anyway, due to "wasted space" in partitioning, don't believe the
sizes in the table.  The standard sizes always do not cover the
partitions (except on the RP0[456]).  Believe the entries in
/etc/disktab or your disk manual, instead.

>Further, it appears that the numbers to use in /etc/mkfs are 1/2 of
>the 'nblocks' number (for "size") along with the standard '3 304' (for
>"m" and "n").  Is this correct?

Berkeley is trying to get people to think in "Kbytes" rather than
sectors or blocks.  So, yes, this is right.

Now for System V.  The only real standard seems to be that the
root partition is 5000 "blocks", and /usr is ... hmmm, that seems
to vary.  Anyway, blocks also vary, being either 512 or 1024
bytes, depending on which file system you look at.  (But mkfs
strongly believes in 1024 bytes.)  Partition 7 is the whole disk.
Whichever of partitions 1-6 are used seem to start at regular
intervals along the disk, or whatever the designer felt reasonable,
and a lot of designers felt different ways.  Each of partitions
1-7, however, include the whole rest of the disc!  This is nice,
because then I have a lot of flexibility in how I set up my file
systems.  This is VERY NASTY, since then I can destroy a later
file system by writing into earlier partitions.  On my RA60's, I
combine partitions 2&3 and 4&5 (6 is empty) into file systems
that are (by no coincidence) exactly the size of my RA81's
partitions.  (You see, I planned this when I wrote the driver.
;-))  BUT, I have to keep this written down somewhere, or I
will do something very nasty like creating another file system
on partition 3, in the middle of an existing one.  I finally
just made those partitions inaccessible (either mode 0 or I
removed them) (and, yes, root can access mode-0 files -- and
WHO LET YOU do everything as root?  you should NEVER use root,
well, unless you really have to.  i have discs owned by bin
and mode 400, and everything works REAL nice.).

I'm beginning to ramble again, aren't I?  Ta.  Happy Thanksgiving
or Christmas or whenever it is wherever you are.
-- 

	Joe Yao		hadron!jsdy@seismo.{CSS.GOV,ARPA,UUCP}