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Re: [RFC/PATCH 0/2] support for CMS-formatted disks on the s/390 archite

From: Stephen Powell
Subject: Re: [RFC/PATCH 0/2] support for CMS-formatted disks on the s/390 architecture
Date: Sat, 17 Apr 2010 11:39:53 -0400 (EDT)

On Fri, 16 Apr 2010 23:46:26 -0400 (EDT), Jonathan Nieder wrote:
> The inimitable Stephen Powell sent something like the two patches
> below to <http://bugs.debian.org/578097>.  It seems that they teach
> the dasd driver in parted to handle CMS-labeled partitions, which are
> a little more flexible and probably more common than the LDL format
> libparted already supports.
> Unfortunately, I do not have a specimen to experiment with, so I
> cannot say if my whitespace tweaks and so on broke the patch.
> Stephen, would it be possible to put up an image of a tiny partition
> to try this out with?

Sure, I'd be happy to.  I'm assuming that you are testing under
Hercules and don't have access to a VM system new enough to
do the new-style (EDF) CMS disk formatting.  (VM/370 Release 6,
which is now in the public domain and can be legally run on
Hercules without a license, is too old for this purpose.  It can
only create the older CDF disks with a block size of 800).
What I don't know are the capabilities of Hercules with regard
to emulating "minidisks".  I am running on a real mainframe and
I don't use Hercules.  What I propose is four 1-cylinder minidisk
images of a 3390 DASD device.  They will be (a) a non-reserved
CMS minidisk with no filesystem (mkfs or mkswap has never been run
on the partition), (b) a non-reserved CMS minidisk with an ext3
filesystem on it, (c) a reserved CMS minidisk with no filesystem,
and (d) a reserved CMS minidisk with an ext3 filesystem.  All
minidisks will be formatted with a block size of 4096.  If
you'd like other scenarios, let me know.
> The support is for reading only because that is what is most needed.
> I don’t imagine there would be any major technical obstacles to
> teaching parted to write these partitions, too.
> Thoughts?

Yes, we really don't need disk reorganization tools on z/VM.  It's
a different environment from PCs, where there is typically one
big disk and partition management is a big deal.  On z/VM, z/VM
itself provides the "partition management" with minidisks.
The single partition on a CMS minidisk cannot be shrunken or expanded
anyway.  By definition, the partition starts either with the
fourth physical block or at the beginning of the CMS reserved file
and proceeds to the end of the minidisk.  Period.  No exceptions.
Moving it or changing its size can only be done by z/VM facilities.
All we really need is for parted to recognize the pre-existing
partition on a CMS minidisk so that the Debian installer can make
use of them.

See http://www.wowway.com/~zlinuxman/diag250.htm for
an idea of the hoops we currently have to jump through to install
Debian for s390 to CMS minidisks.  These hoops will still be
necessary if the target minidisks are on FBA DASD, as my enhancement
patches are only for CKD DASD.  But at least for customers using
CKD DASD this problem will be solved.  I don't have any FBA DASD to
test with, or I might have attempted a patch for that too.  If
Hercules can emulate FBA DASD, and you can get CMS minidisk images
for FBA DASD from somewhere else, you're welcome to try that too,
but I can't test it for you.

  .''`.     Stephen Powell    
 : :'  :
 `. `'`

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