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[dann frazier <dann.frazier@xxxxxxxxxxxxx>]

** Bug watch added: Debian Bug tracker #944676
   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=944676

** Also affects: mdadm (Debian) via
   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=944676
   Importance: Unknown
       Status: Unknown

** Also affects: ubuntu-release-notes
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1850540

Title:
  multi-zone raid0 corruption

Status in Release Notes for Ubuntu:
  New
Status in linux package in Ubuntu:
  Confirmed
Status in mdadm package in Ubuntu:
  Confirmed
Status in linux source package in Precise:
  New
Status in mdadm source package in Precise:
  New
Status in linux source package in Trusty:
  Confirmed
Status in mdadm source package in Trusty:
  Confirmed
Status in linux source package in Xenial:
  Confirmed
Status in mdadm source package in Xenial:
  Confirmed
Status in linux source package in Bionic:
  Confirmed
Status in mdadm source package in Bionic:
  Confirmed
Status in linux source package in Disco:
  Confirmed
Status in mdadm source package in Disco:
  Confirmed
Status in linux source package in Eoan:
  Confirmed
Status in mdadm source package in Eoan:
  Confirmed
Status in linux source package in Focal:
  Confirmed
Status in mdadm source package in Focal:
  Confirmed
Status in mdadm package in Debian:
  Unknown

Bug description:
  Bug 1849682 tracks the temporarily revert of the fix for this issue,
  while this bug tracks the re-application of that fix once we have a
  full solution.

  Fix checklist:
  [ ] Restore c84a1372df929 md/raid0: avoid RAID0 data corruption due to layout confusion.
  [ ] Also apply these fixes:
      33f2c35a54dfd md: add feature flag MD_FEATURE_RAID0_LAYOUT
      3874d73e06c9b md/raid0: fix warning message for parameter default_layout
  [ ] If upstream, include https://marc.info/?l=linux-raid&m=157239231220119&w=2
  [ ] mdadm update (see Comment #2)
  [ ] Packaging work to detect/aide admin before reboot

  Users of RAID0 arrays are susceptible to a corruption issue if:
   - The members of the RAID array are not all the same size[*]
   - Data has been written to the array while running kernels < 3.14 *and* >= 3.14.

  This is because of an change in v3.14 that accidentally changed how data was written - as described in the upstream commit message:
  https://github.com/torvalds/linux/commit/c84a1372df929033cb1a0441fb57bd3932f39ac9

  That change has been applied to stable, but we reverted it to fix
  1849682 until we have a full solution ready.

  To summarize, upstream is dealing with this by adding a versioned
  layout in v5.4, and that is being backported to stable kernels - which
  is why we're now seeing it. Layout version 1 is the pre-3.14 layout,
  version 2 is post 3.14. Mixing version 1 & version 2 layouts can cause
  corruption. However, until an mdadm exists that is able to set a
  layout in the array, there's no way for the kernel to know which
  version(s) was used to write the existing data. This undefined mode is
  considered "Version 0", and the kernel will now refuse to start these
  arrays w/o user intervention.

  The user experience is pretty awful here. A user upgrades to the next
  SRU and all of a sudden their system stops at an (initramfs) prompt. A
  clueful user can spot something like the following in dmesg:

  Here's the message which , as you can see from the log in Comment #1,
  is hidden in a ton of other messages:

  [ 72.720232] md/raid0:md0: cannot assemble multi-zone RAID0 with default_layout setting
  [ 72.728149] md/raid0: please set raid.default_layout to 1 or 2
  [ 72.733979] md: pers->run() failed ...
  mdadm: failed to start array /dev/md0: Unknown error 524

  What that is trying to say is that you should determine if your data -
  specifically the data toward the end of your array - was most likely
  written with a pre-3.14 or post-3.14 kernel. Based on that, reboot
  with the kernel parameter raid0.default_layout=1 or
  raid0.default_layout=2 on the kernel command line. And note it should
  be *raid0.default_layout* not *raid.default_layout* as the message
  says - a fix for that message is now queued for stable:

  https://github.com/torvalds/linux/commit/3874d73e06c9b9dc15de0b7382fc223986d75571)

  IMHO, we should work with upstream to create a web page that clearly
  walks the user through this process, and update the error message to
  point to that page. I'd also like to see if we can detect this problem
  *before* the user reboots (debconf?) and help the user fix things.
  e.g. "We detected that you have RAID0 arrays that maybe susceptible to
  a corruption problem", guide the user to choosing a layout, and update
  the mdadm initramfs hook to poke the answer in via sysfs before
  starting the array on reboot.

  Note that it also seems like we should investigate backporting this to
  < 3.14 kernels. Imagine a user switching between the trusty HWE kernel
  and the GA kernel.

  References from users of other distros:
  https://blog.icod.de/2019/10/10/caution-kernel-5-3-4-and-raid0-default_layout/
  https://www.linuxquestions.org/questions/linux-general-1/raid-arrays-not-assembling-4175662774/

  [*] Which surprisingly is not the case reported in this bug - the user
  here had a raid0 of 8 identically-sized devices. I suspect there's a
  bug in the detection code somewhere.

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