Bcachefs Multi-Device Users Should Avoid Linux 6.7: "A Really Horific Bug"

If you were feeling adventurous and began using the Bcachefs file-system upon its introduction in Linux 6.7 mainline and using it for a multi-device setup, you are best off upgrading to Linux 6.8 as soon as possible due to known issues with the code in v6.7.

Bcachefs lead developer Kent Overstreet issued a clear warning on Friday night that Bcachefs multi-device users should move off Linux 6.7 ASAP. Kent wrote on the bcachefs mailing list:

there's a bug in 6.7 with filesystems that are mid upgrade and then get downgraded not getting marked in the superblock as downgraded, and this translates to a really horrific bug in splitbrain detection when the old version isn't updating member sequence nmubers and you go back to the new version - this results in every device being kicked out of the fs.

and our backports are not being picked up by the stable team, so - do not run 6.7, switch to 6.8 immediately, running 6.7 with new tools will trigger it.

if you are affected:

- 6.9 (once Linus merges) will have a new no_splitbrain_check option, which runs the splitbrain checks in dry mode and won't kick your devices out

- we have new repair code landing soon that can recover from missing/unreadable btree roots by scanning the entire device(s) for btree nodes (which, fortunately, we have sufficient metadata in btree node headers to do safely; reiserfs famously did not). i've seen some crazy corruption resulting from this, but it might still be recoverable

Kent's been trying to get the fixes picked up for a Linux 6.7 stable point release to no avail amid ongoing disputes between the stable maintainers over the processes. But long story short, if you are using Bcachefs multi device configurations, move past Linux 6.7 right away.