[Bug 1159464] Re: make persistent storage pathnames fully configurable

[Ian Bruce]

It seems that substring replacement is not supported in Bourne shell:

    y=${x//aaa/bbb}

-- does not work, and therefore the patch which was provided above
is invalid. See below for a new patch which uses the standard utility
program "tr" to accomplish this purpose.

After rebuilding a patched initrd, these changes have been tested
with the following distributions:

bodhi-2.3.0
linuxmint-14.1-mate-dvd
linuxmint-14.1-cinnamon-dvd
ubuntu-12.10-desktop
kubuntu-12.10-desktop
xubuntu-12.10-desktop

All six ISOs were installed on a single USB flashdrive, and could be
rebooted with shared persistent user data, and unshared persistent
system data, as described above. Further testing is required, but
currently this patch seems to be a complete solution to the problem.

-- Ian Bruce

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https://bugs.launchpad.net/bugs/1159464

Title:
  make persistent storage pathnames fully configurable

Status in “casper” package in Ubuntu:
  New

Bug description:
  I am working on a project which allows the installation of multiple Linux live ISO images
  on a single USB flashdrive.

  http://sourceforge.net/projects/multibootusb/

  We would like to make use of the Ubuntu live ISO feature wherein persistent changes
  to the live ISO filesystem may be saved on the same USB flashdrive, and restored across
  reboots.

  From inspection of the casper source code, it appears that persistent data may be stored
  in two different filesystem images; one for system data, and the other for user data.

  Since our project expects that several bootable distributions will coexist on the same
  flashdrive, it would clearly be a bad thing if changes to one operating system affected
  the others, and thus persistent system data ought NOT to be shared between them.
  However, it would be quite inconvenient for users if files that they had saved while
  running one distribution were not available from another, and therefore persistent
  user data SHOULD be shared between the installed distributions.

  Unfortunately, it appears that it is not currently possible to accomplish this outcome,
  because the names of the persistent filesystem images are not configurable; they
  must be "casper-rw" for the system data, and "home-rw" for the user data. The only
  thing that may be configured is the directory within which these two filesystem images
  reside. Thus either both filesystems, or neither, may be shared between different
  distributions. It is not possible to have one shared and the other unshared.

  The attached patch removes this limitation, by making the full pathnames of the two
  persistent filesystem images independently configurable. It does this by introducing
  two new initrd boot options: "persistent-root=" and "persistent-home=". These were
  chosen in conformance with the existing boot option "persistent-path=", whose function
  remains unchanged if neither of the new options is used (except that it is no longer
  necessary to also specify the flag option "persistent" when it is present).

  It is intended that the new options will specify the full pathnames of the persistent
  filesystem images (or possibly only one of them), relative to the root directory of
  some attached device, as does the "persistent-path=" option. If they are not used,
  the code functions exactly as it did before, ensuring full backward compatibility with
  other interacting software. There is some amount of overlap with the code for "snapshots",
  which I did not entirely understand, but I took care that its function would be completely
  unaffected.

  I would point out that the current code contains this comment:

      # FIXME: add a custom mounts configurable system

  I believe that this is exactly what I have just done.

  *******

  I am aware that a new version of Ubuntu will be released soon, according to the standard
  schedule. Because this patch is extremely small and self-contained, yet provides a significant
  improvement in functionality, as described above, I would ask that it be considered for the
  upcoming release. I should say that my description of multiple (Ubuntu-based) distributions
  coexisting on the same flashdrive, and sharing the same set of persistent user data, is not
  hypothetical. It is already working perfectly in our development version. We cannot yet
  enable (unshared) persistent system data, for reasons already discussed, but this will also
  be functional as soon as this patch is accepted into released ISO images.

  Thanks for your consideration.

  
  -- Ian Bruce

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