[Bug 1559193] [NEW] disk-detect/s390-dasd/s390-zfcp: Restructure installer and put DASD and FCP configuration into disk detection

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== Comment: #1 - Hendrik Brueckner <brueckner at de.ibm.com> - 2016-03-18 09:23:45 ==
On Linux on z Systems, there are two major disk storage environments,
the direct-attached storage disk (DASD) and SCSI over Fibre-Channel.

There are the s390-dasd and s390-zfcp d-i modules to configure and
enable DASDs and FCP devices.  Note that on s390x, disks are not
available by default to Linux and must be enabled in advance.

The s390-dasd and s390-zfcp both provide the harddrive-detection
dependency and, thus, each could fulfill the dependency to silenty
ignore the other.  That behavior does not allow to mix DASDs and
SCSI disk on single installation (except you call both manually,
for example, in the expert mode).

To improve and provide a "guided" flow, I have split the harddrive
detection dependency for s390-dasd and s390-zfcp as follows:

- s390-dasd provides harddrive-detection-dasd
- s390-zfcp provides harddrive-detection-zfcp

disk-detect depends on
  -> harddrive-detection-dasd
  -> harddrive-detection-zfcp

and continues to provide the harddrive-detection.

With this split, the guided installation will install disk-detect to
solve the harddrive-detection dependency.  In turn, disk-detect will
then rely on the s390-dasd and s390-zfcp d-i modules to provide DASD
and FC-attached SCSI disks.  If both modules fail, the user perceives
the default disk-detect behavior, for example, users might configure
iSCSI.

The other nice benefit of this dependency split is the seamlessly
enablement of multipath with disk-detect (through preseeding).
For s390, multipathing should be always considered when SCSI is used.
I probably will extend the s390-zfcp module to set disk-detect's
multipath debconf variable for usability.


There are already Debian bug reports with patches to provide this solution:

disk-detect: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=818586
s390-dasd: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=818591
s390-zfcp: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=818592

** Affects: hw-detect (Ubuntu)
     Importance: Undecided
     Assignee: Dimitri John Ledkov (xnox)
         Status: New


** Tags: architecture-s39064 bot-comment bugnameltc-139323 severity-high targetmilestone-inin1604
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disk-detect/s390-dasd/s390-zfcp: Restructure installer and put DASD and FCP configuration into disk detection
https://bugs.launchpad.net/bugs/1559193
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