[Bug 1601997] Re: Ubuntu 16.10+ installer uses ext4 feature 'metadata_csum' which is incompatible with older (LTS) e2fsprogs

[Simon Iremonger]

Can we pay-attention, to this thread (now) being about considering the
feature-flags 'used by default' in mke2fs.conf, in consideration to
18.04 --  [linked bug for e2fsprogs].

We know massive 'compatibility'/portability benefit of formatting the
same as previous-LTS by default, as the required e2fsprogs was only
introduced 16.10, and the needed kernel was only in 16.04 (and 14.04
-HWE-kernel-update.).  NB: Looks like there is a similar issue with
GRUB2 which affects dual-booters, grub2 2.02beta3 may be needed to
support 64bit FS.  If so, again Xenial isn't compatible to detect an
18.04 in multi-boot-menu.

I'd like tytso to comment on the downsides of formatting without
64bit,metadata_csum, more specifically.  From what I can (see) this only
loses a little extra checksumming-integrity (against bad disks, which
we've never needed before??), and the 64bit feature appears only to be
needed to strengthen these checksums or support >16TB disks (bearing in
mind auto_64bit_support can still be used).  NB: Is this correct, -OR-
is there likely going to be future annoyances with 'other' ext4
features-to-come which will ALSO expect 64bit formatting?.

My suggestion is, this hasn't been sorted-out enough in the ubuntu-
world, and 64bit,metadata_csum should be disabled-by-default for 18.04
and backporting grub/e2fsprogs/etc where relevant should happen as well
[see other thread, possibly another may be created for grub2].

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https://bugs.launchpad.net/bugs/1601997

Title:
  Ubuntu 16.10+ installer uses ext4 feature 'metadata_csum' which is
  incompatible with older (LTS) e2fsprogs

Status in e2fsprogs package in Ubuntu:
  Triaged

Bug description:
  Ubuntu 16.10 installer sets metadata_csum option on ext4 partition
  which is incompatible with other LTS Ubuntu versions (12.04 LTS, 14.04
  LTS, 16.04 LTS).

  Steps to reproduce:
  1. Download Ubuntu 16.10 installation media.
  2. Install Ubuntu.
  3. Try to do fsck -fy /dev/sdX1 from other supported Ubuntu distro.

  Expected results:
  User can check and fix errors on ext4 filesystem, created on Ubuntu 16.10.

  Actual results:
  User can not check and fix errors on ext4 filesystem because of lack of 'metadata_csum' option in previous LTS Ubuntu versions.
  The only one working solution was to scan from 16.10 live install media.

  Note:
  it is known, that Dan Watkins disabled metadata_csum when creating ext4 filesystems ( see http://bazaar.launchpad.net/~daniel-thewatkins/maas-images/fix-yakkety-builds/revision/305 ). It is good solution.

  ProblemType: Bug
  DistroRelease: Ubuntu 16.10
  Package: e2fsprogs 1.43.1-1
  ProcVersionSignature: Ubuntu 4.4.0-30.49-generic 4.4.13
  Uname: Linux 4.4.0-30-generic i686
  ApportVersion: 2.20.2-0ubuntu1
  Architecture: i386
  CurrentDesktop: Unity
  Date: Mon Jul 11 23:42:49 2016
  SourcePackage: e2fsprogs
  UpgradeStatus: No upgrade log present (probably fresh install)

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