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

[Jim Meyering]

Stephen Powell wrote:
...
> Yes, that is absolutely correct.  An LDL- or CMS-format disk,
> by definition, has one and only one partition, and it is
> non-modifiable.  By that I mean that the size and location
> of the partition can only be changed by reformatting the disk
> with a different block size, or for a CMS disk, by reserving
> the disk if it has just been formatted by CMS, has not been written
> to by Linux at all, and has not been reserved since formatting.
> Other than that, the partition is non-modifiable.  The partition
> is writable.  That is, you can create a file system or a
> swap space on it.  But the size and location of the one-and-
> only-one partition cannot be changed.
>
> The end of the
> metadata (treated like a VTOC) is, by definition, the block
> which precedes the start of the first (and only) partition.
> The start of the metadata is the first block (block 0).
> The disk contains no free space, by definition.
> Formerly, start was set to 24 and vtoc_end was set to 23,
> hard coded.  That is correct only for a block size of 4096,
> and for a CMS format disk, only if it is not reserved.
>
>> If you really do want to use the start sector of partition #1,
>> we can pursue that separately.

Thanks for all the explanation.
It sounds like you really do want to use that p1 start sector number.
You might want to use a new function to return that number, e.g.,

static PedSector
partition_1_start_sector (const PedDisk *disk)
{
  const PedPartition *part = ped_disk_get_partition(disk, 1);
  assert (part);
  const LinuxSpecific *arch_specific = LINUX_SPECIFIC (disk->dev);
  PedSector start = (part->geom.start
                     * disk->dev->sector_size
                     / arch_specific->real_sector_size
                     / disk->dev->hw_geom.sectors);
  return start;
}

[Beware, I've just typed that in, and didn't test at all.
 It might not even compile.
 The computation is the same as the one done in the dasd_write loop. ]