Minimum inode size should be raised to 256 for forward compatibility (Y2038)

[ABCMoNa]

mke2fs in all supported Ubuntu releases to date creates 'small' ext4 file systems with an inode size of 128. This is said to be insufficient to store time stamps referring to year 2038 and later (when the Unix epoch exceeds a signed 32-bit integer - https://en.wikipedia.org/wiki/Year_2038_problem). When such file systems are mounted by the kernel, such as a /boot file system created by the Ubuntu 20.04.0 Desktop installer (ubiquity 20.04.15), the kernel will warn about this fact:

  kernel: ext4 filesystem being mounted at /boot supports timestamps until 2038 (0x7fffffff)

Cf. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=953494

Admittedly, that's another 18 years, by when most systems installed now will be no longer in use, or will have been reinstalled.

Also, on a /boot file system the 5.4.0-33-generic #37 Ubuntu (20.04) kernel warned about this, I was able to
  $ touch -t 203812312359 /tmp/test
and got
  $ ls -l /tmp/test
  -rw-r--r-- 1 root root 0 Dez 31 2038 /tmp/test
so this warning may not always apply, or patches may have been applied to handle this situation more gracefully already.

Ubuntu 20.04 comes with e2fsprogs 1.45.5-2ubuntu1. Theodore Y. Ts'o has submitted patches for Debian's 1.45.6-1 - but (strictly speaking based on the changelog) these seem to be limited to documentation changes.

Still, creating such file systems may seem undesirable in terms of forward compatibility. I therefore suggest to change the minimum inode size for mke2fs, as defined in /etc/mke2fs.conf, to 256 for all file systems which do not already use this value (maybe with the exception of floppies). That is, I suggest the following change to Ubuntu 20.04's /etc/mke2fs.conf:

Before change:
        small = {
                inode_size = 128
                inode_ratio = 4096
        }

After change:
        small = {
                inode_ratio = 4096
        }

This assumes that the default inode size is already set to 256 further above.

As a WORKAROUND, after backing up the file system (this command is not well tested, according to the lead ext4 file system developer), users can unmount the affected file system and run
  tune2fs -I 256 /path/to/block_device
where /path/to/block_device should be replaced by the path to the block device which contains the file system the kernel reports the warning for. For example, for the file system mounted at /boot, the
  findmnt /boot
command would return the block device path in the SOURCE column.

Please note I prefer not to use apport for this bug report, I believe it is already sufficiently complete.

[ABCMoNa (abcmona)]

Actually this is not an installer issue, just indirectly.

[Theodore Ts'o (tytso)]

This relatively simple to change the default inode size in all cases. Look at /etc/mke2fs.conf, which is in the sources as misc/mke2fs.conf.in. Find the lines where the inode_size is set to 128, and change it to be 256. (This might or might cause problems on Hurd; since I'm not sure whether the GNU Hurd can support modern file systems with 256 byte inodes. Heck, the GNU Hurd implementation doesn't support journalling or extents last I checked.)

This will double the overhead of ext4 file systems, especially on small file systems. This is why the default is set the way that it is. For really tiny thumb drives (especially the cheap ones found in the checkout aisle of the Micro Center), they are very likely to self-destruct within a few months, never mind 18 years. And users might get cranky if the amount of usable space is decreased.

It may be that for the installer, it will want to hard code the use of a larger inode size, or specify a file system usage type hint via the -T option which could trigger a different default in mke2fs.conf. This is fundamentally a policy question, not a technical issue --- and different distributions may have different opinions about what the defaults should be; this is why we've made it easy to customize the defaults should be in misc/mke2fs.conf.in in the sources, as opposed to hard-coding it in the misc/mke2fs.c source file.

[Janne Blomqvist (blomqvist-janne)]

It seems the default was fixed in upstream at https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/commit/misc/mke2fs.conf.in?id=a23b50cdb55cb826b8745cbc37429c93f7b60c66

Also, it seems Ubuntu 22.04 has a version of e2fsprogs (1.46.5-2ubuntu1) which includes this change. Thus I think this bug can be closed as resolved.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.