[Bug 1365874] Re: Ubuntu 12.04 LTS, 14.04 LTS, 16.04 LTS do not support ext4 metadata checksumming

[Theodore Ts'o]

The Metadata_csum feature is a RO_COMPAT feature.   Hence, grub2 and os-
prober shouldn't care about this feature (if properly implemented).
This is because the guarntee is that even if the kernel (or userspace
application directly accessing the file system) doesn't know about a bit
in the RO_COMPAT bitmask, it is safe to access the file system in read-
only mode.

Now, the 64-bit feature is an INCOMPAT feature.   This means that if the
kernel (or user-space program directly accessing the file sytem) doesn't
understand a bit in the INCOMPAT feature set, it is *not* safe for it to
try to understand the file system.    This feature was only enabled in
e2fsprogs 1.43.x if the file system was larger than 16TB.   JHowever, by
e2fsprogs 1.44.0, it is turned on by default even for file systems
smaller than 16TB.  That's because it was expected that by now, we had
given grub2 and os-prober enough time to get with the program --- and if
you create a file system without the 64-bit feature, it is not possible
to online resize it beyond 16TB.   So this is why we turn on the 64-bit
feature out of the box with e2fsprogs 1.44.0 (there are failure modes
with leaving it turned off for the sake of backwards compatibility with
antique software versions).    However, if a enterprise distribution
decides that backwards compatibility is more important new features, it
can ship e2fsprogs with an edited version of misc/mke2fs.conf.in.
Feel free to turn off 64-bit if you think breaking the 15TB->16TB online
resize is an acceptable consequence about the 16.04 vs 18.04 multi-
booting issue.   That's why the customers pay the enterprise distro
providers the big bucks.   :-)

Or get really frustrated with the enterprise distro providers.
Probably both.  :-)

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https://bugs.launchpad.net/bugs/1365874

Title:
  Ubuntu 12.04 LTS, 14.04 LTS, 16.04 LTS do not support ext4 metadata
  checksumming

Status in e2fsprogs package in Ubuntu:
  Triaged

Bug description:
  In the Trusty release notes "Metadata checksumming" is listed as one
  of the tech highlights of kernel 3.13.

  https://wiki.ubuntu.com/TrustyTahr/ReleaseNotes#New_features_in_14.04_LTS

  However, the userland tools do not support this kernel feature. To the
  best of my knowledge this will be supported in 1.43, and won't be
  backported to 1.42.

  IMHO this is very misleading. It's like a car salesperson sold you a
  sports car with an V8 engine. After you drove it home, you opened the
  hood and realized only 6 cylinders are working because 2 of the spark
  plugs were not included, and you have to buy aftermarket ones.

  <rant>BTW, I've been following the development of e2fsprogs for over a
  year. 1.43  release has been in limbo for God knows how long :(
  </rant>

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