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The kernel wiki page for ext4 metadata checksums is marked "last modified on 22 October 2013". I couldn't find more recent status information on this feature, except for this question from 2019, which suggests to disable it and the related 64bit feature. One of the answers there claims that the 64bit feature "is not well-tested", but I'm not sure if that's correct. There was no basis quoted for that claim, and given the prevalence of 64-bit systems in recent years, it sounds questionable. I do know metadata_csum depends on 64bit for full checksums.

My question:
As of 2020+, are these two related features (metadata_csum and 64bit) considered stable and safe? How well tested are they, really? Are there any important bugs, gotchas, or failure modes that should be considered before enabling these?

MichaelK
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  • Your systems *e2fsprogs* man pages (`man 8 tune2fs`) hopefully contain *much* more recent information than that (you can likely trust the userland programs to be paired with compatible kernel versions: the Debian maintainer is also kernel subsystem maintainer) – anx Jun 17 '21 at 21:22

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metadata_csum should be stable, as it has found its way in RHEL8

64bit is the same; from ext4 man page:

This feature is set automatically, as needed

shodanshok
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  • That sounds outdated, see my answers. resize2fs *errors out* instead of setting it automatically, and mke2fs *unconditionally* uses the command line setting or the default instead of deferring to necessity. – anx Jun 17 '21 at 21:35
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A certain Theodore Ts'o has, back in in 2015, enabled both feature by default for filesystems created with version e2fsprogs 1.43. Before that, it was only enabled when needed.

Mke2fs will now create file systems with the metadata_csum and 64bit features enabled by default. e2fsprogs/doc/RelNotes/v1.43.0.txt

That default having found its way into Ubuntu 18.04 (that is the LTS release before the current one) should mean that at least in any setup supported and used with Ubuntu, it is widely tested with kernel versions starting with 4.15. If there were serious issues left, we would know by now.

anx
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  • example of a [noteworthy outstanding bug](https://bugs.launchpad.net/qemu/+bug/1805913) only affecting unusual setups – anx Jun 17 '21 at 22:06