Comment 20 for bug 1365874

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. :-)