software RAID in FAI

[Eric Mumpower]

> You can write a partition hook which includes all your manual commands
> to create the RAID system. The call the default task_partition or skip
> it if not needed.

Which is more or less what I've done. However, I have no desire to
re-implement much of the layout/partitioning algorithms already included in
setup_harddisks. Plus, there is a great deal of flexibility to be gained by
integrating the setup of a RAID with your setup_harddisks script.

So far, I've patched setup_harddisks to allow a partition type "softraid"
which is not formatted and which is given the Linux-raid-autodetect
partition ID. My partitioning hook script (partition.RAID) wraps around
setup_harddisks, re-using its partitioning/sfdisk logic, and then creating
the raidtab, the RAID volumes, disk_var.sh, and the fstab.

While this is more or less adequate, there are two points on which it is
fragile and thus fails to be a good implementation:

 * Either my partition.RAID script needs to know which partition numbers
    setup_harddisks used for the requested partitions (since the config
    format for setup_harddisks quite wisely doesn't allow that information
    to be specified), or the raidtab-creating functionality needs to be
    integrated into setup_harddisks. As I don't expect you to feel that
    this feature is of sufficiently broad appeal, I assume that you would
    prefer to incorporate some mechanism for the first option I described.

 * setup_harddisks does not allow you to specify that two partitions on
    separate disks be *precisely* equal numbers of sectors. This is a
    desirable option for someone creating partitions for use by a software
    RAID.

I would be happy to:
 * propose implementations along whichever lines you suggest
 * once a direction of implementation is agreed-upon, to offer patches to
    setup_harddisks implementing these features

Can you offer any guidance?