e2fsprogs generates filesystems that might render installs of older releases unbootable (due to GRUB)

Current versions of e2fsprogs in Debian have enabled the metadata_csum_seed / large_dir features, but as Debian bug #1031364 explains, this makes the filesystem unknown for GRUB2. The e2fsprogs maintainer points out that the fix for GRUB2 has been available for almost two years, but no stable releases of GRUB2 have been released since then.

The GRUB2 maintainers in Debian have included the patch, so current versions of GRUB2 (>= 2.06-8) will boot correctly. However, vmdb2 can be run in systems running a newer set of binaries, but building an older system — which, if uses vmdb2, will be rendered unbootable.

AIUI, tune2fs can be used to remove the metadata_csum_seed feature from an already created filesystem, but large_dir has to be specified at filesystem creation time — tune2fs(8) mentions:

large_dir
    Increase the limit on the number of files per directory.  Tune2fs currently
    only supports setting this file system feature.