[Bug 1561606] [NEW] Booting support with multiple squashfs files

[Daniel]

Public bug reported:

AFAIK, Casper supports booting when there is more than one squashfs file
in the casper directory. At least, this _was_ supported under Ubuntu
trusty which used aufs as union file system. Then there came utopic,
wily, vivid which switched to overlayfs. This file system didn't support
layering more than one ro sources in its early versions so that this
feature didn't work anymore. The code in the casper script looks like a
work-around about this by stacked mounting multiple overlayfs.
Unfortunately, I've never seen a version of overlayfs which supported
another overlayfs as source.

Since kernel 3.19, overlayfs officially supports multiple layers
(http://www.phoronix.com/scan.php?page=news_item&px=MTg1Nzg), so it's
time to rewrite this piece of code in the casper script.

Fully-working patch attached.

** Affects: casper (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1561606

Title:
  Booting support with multiple squashfs files

Status in casper package in Ubuntu:
  New

Bug description:
  AFAIK, Casper supports booting when there is more than one squashfs
  file in the casper directory. At least, this _was_ supported under
  Ubuntu trusty which used aufs as union file system. Then there came
  utopic, wily, vivid which switched to overlayfs. This file system
  didn't support layering more than one ro sources in its early versions
  so that this feature didn't work anymore. The code in the casper
  script looks like a work-around about this by stacked mounting
  multiple overlayfs. Unfortunately, I've never seen a version of
  overlayfs which supported another overlayfs as source.

  Since kernel 3.19, overlayfs officially supports multiple layers
  (http://www.phoronix.com/scan.php?page=news_item&px=MTg1Nzg), so it's
  time to rewrite this piece of code in the casper script.

  Fully-working patch attached.

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