Comment 11 for bug 1601997

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].