Re: Max usable space on ProDOS hard drive?

[dempson@actrix.gen.nz (David Empson)]

Mister Boffo <mister_boffo@hotmail.com> wrote:

> I have an Apple High-Speed SCSI lying around.
> And an APS 700 meg SCSI drive.
> 
> So, can an Apple II really make use of all 700 megs or is
> there some sort of "hard limit" to what it can access?

Under ProDOS-8, there is an absolute limit to the number of partitions
you can access, which varies somewhat depending on which SCSI card you
have, which ProDOS-8 version you are running, the slot settings for the
IIgs, and what cards are installed in slots set to "Your Card".

Under GS/OS, there is a higher limit on the total number of partitions,
which again varies depending on your SCSI card.

ProDOS-8 can only access ProDOS-formatted partitions on the hard drive.
GS/OS is also able to read and write HFS partitions (but cannot boot
from one).  In general, with a large hard drive on a IIgs, the best
policy is to create a number of 32 megabyte ProDOS-8 partitions,
according to your requirements (including the first partition), and one
or more HFS partitions for the rest of the drive.

> I think ProDOS can create up to 32 meg partions.

Correct.  The maximum space on a single ProDOS-8 partition is 32
megabytes (actually 512 bytes less than this).

Here are some of the limits you will run into on the number of
partitions.

1. Firmware limits imposed by the card.  (These only affect ProDOS-8,
not GS/OS.)

- If you have an original Apple SCSI card, you cannot access more than 7
volumes under ProDOS-8 (counting all partitions on all drives connected
to the card).

- If you have an Apple High-Speed SCSI card, the corresponding hard
limit under ProDOS-8 is a theoretical 110 partitions.  (The practical
limit is much lower.)

- If you have a RamFast SCSI card, I don't know if there is a fixed
upper limit.

2. Limits imposed by ProDOS-8.

- If you are running ProDOS 1.0 or 1.1, you can only access two volumes
per slot.

- If you are running ProDOS-8 1.2 through 1.9, you can access two
volumes per slot, except in the case of slot 5, where you can access
four volumes, provided the card supports this (Apple's SCSI cards do).
The additional two drives are mapped to slot 2, but this can only occur
if slot 2 does not contain anything which ProDOS-8 regards as a disk
controller (including SCSI cards and some types of RAM card).

- If you are running ProDOS-8 2.0 or later, and the card supports the
SmartPort firmware interface (both Apple cards do, but the RamFast
doesn't), ProDOS can remap additional partitions to unused slots.  For
the purposes of this remapping, ProDOS regards a slot as used if it
contains somethink which looks like a disk controller (same rule as
above).  If the slot has a single drive (e.g. a RAM card or a floppy
disk controller with one drive connected) you cannot use the second unit
number.  You also need to allow for S3,D2 which is always the /RAM
volume (64K RAM disk in auxiliary memory).  ProDOS will always access
the partitions in ascending order on the drive, until it runs out of
available unit numbers.  The maximum theoretical limit which can be
achieved here is 13 partitions, but that would only be possible if you
had no floppy disk controllers.

- The RamFast card has its own mechanism for remapping partitions, which
allows you to select active partitions from a menu.  It has greater
flexibility than the Apple SCSI cards, and may allow you to access more
than 13 partitions.

3. Limits imposed by GS/OS.

- For either type of Apple SCSI card, GS/OS is limited to a total of 63
volumes from each type of device (separate limit for hard drives and
CD-ROM drives), counting all drives of the same type connected to all
Apple SCSI cards installed in the machine.

- I don't know what upper limit is imposed by the GS/OS RamFast driver,
but it cannot be any higher than 63.  I think it uses a single driver
for all device types, so the limit would apply to the total number of
partitions counting all hard drives and CD-ROM drives on all RamFast
cards.

> So I guess I would need to create 21 32-meg partitions to use 672 megs of
> space, then one partition of 28 megs to get the last bit used. Can ProDOS
> handle 22 disk volumes like this?

No.

> If it can't make use of all the space is it possible for
> ProDOS volumes to coexist with Mac, PC, or other types of
> SCSI partitions?

It is possible to share a hard drive between a IIgs and Mac, but isn't
particularly easy to set up and get working reliably, unless you are
dealing with an older Mac.

It is very hard to share a hard drive between a IIgs (or Mac) and a PC,
because they use different partitioning schemes.  With a lot of careful
manual setup, it may be possible to create a hard drive with mutually
exclusive partitions (like the "Zip Tools" disk which used to be
supplied with a Zip drive).