Bcachefs Moves To External DKMS Module In Linux Kernel 6.18

As you may already know, the Bcachefs file system is moving out of the main Linux kernel and will now ship as an external DKMS module. Kent Overstreet, the Bcachefs developer, assures us that for most users, things should stay very much the same.

TL;DR

• After continuous dispute with Bcachefs developer, Linus Torvalds explicitly stated his intention to 'part ways in the 6.17 merge window' with Bcachefs development.
• Consequently, the MAINTAINERS file in the Linux kernel source tree was updated to mark Bcachefs as "Externally maintained" instead of "Supported".
• Kent Overstreet, the Bcachefs developer, subsequently outlined plans to ship Bcachefs as a DKMS module going forward.

What's Happening with Bcachefs?

Last month (August 2025), Linus Torvalds decided to mark Bcachefs as "Externally maintained" in the Linux kernel starting with the 6.17 merge window.

This means its development now takes place outside the main kernel tree.

Consequently, Kent Overstreet plans to distribute Bcachefs as a DKMS (Dynamic Kernel Module Support) module moving forward.

Many users might think this sounds like a big change, but Kent truly believes "very little should change for end users" once the DKMS packages are fully ready.

Because DKMS modules work just like any other kernel module, you can still include them in your initramfs. This ensures your Bcachefs root filesystem continues to function perfectly.

When Will These Changes Take Effect?

Initially, some distributions, like openSUSE, planned to disable Bcachefs as early as kernel 6.17.

However, Kent asked them to hold off for one more release, until kernel 6.18, giving everyone time to prepare the DKMS version. OpenSUSE kernel maintainers agreed to this plan.

The current Bcachefs 6.16 version is very solid and unaffected. You will not lose immediate access to your filesystem.

Kent states that the 6.16 release has seen "zero new critical bug reports". Fixes since then have mostly addressed minor performance tweaks or issues only appearing on test dashboards.

The most significant bug was a repair issue that still allowed successful data repair and mounting without you doing anything.

In even better news, Kent plans to remove the "experimental" label from Bcachefs when kernel 6.18 arrives. He works hard on rebalance_v2 and other bugs to achieve this important milestone.

Upcoming Bcachefs Changes: It Moves to External DKMS Module in Linux Kernel

Bcachefs is now shifting to an external DKMS module in Linux kernel 6.18 after becoming "externally maintained".

While the transition aims for a smooth experience, some differences will depend on your Linux distribution:

1. DKMS and Initramfs

DKMS packages will allow Bcachefs to load dynamically, even if it is not directly included in your main kernel. This is a common method for out-of-tree modules and supports root filesystems without issues.

2. Distribution Specifics

• openSUSE users: You may encounter some specific challenges. openSUSE does not prefer DKMS because it often struggles with rebuilding modules across kernel updates, especially with Tumbleweed's changing kernel API. However, Kent is flexible. He is confident that if a KMP (Kernel Module Package) approach suits openSUSE better, "someone can figure it out".
• Arch and NixOS users: You are in a good spot! These distributions usually handle bcachefs-tools packaging very well.
• Fedora users: You also have a maintained bcachefs-tools package available.
• Other distributions: Kent and the community are still assessing the packaging situation for other distributions.

3. Keeping bcachefs-tools Updated

Getting an up-to-date bcachefs-tools package is now more critical. This has been a lower priority for some distributions in the past, leading to outdated or even removed packages (like in Debian).

Efforts are underway to fix this, perhaps by getting it back into Debian's experimental branch or setting up a PPA.

4. For the DIY Enthusiast

If you prefer compiling your own kernels or want to avoid DKMS complexities, Kent will continue to maintain his git trees, basing them on .0 kernel releases. This gives you another solid option.

Why Bcachefs is not Merged in Linux Kernel 6.17?

One of the most talked-about parts of the Kernel 6.17 RC1, especially in the community discussions, is bcachefs.

Bcachefs maintainer, Kent Overstreet, submitted a pull request for 6.17-rc1 that focuses heavily on bug fixes. He noted that bcachefs is in a "hard freeze" for new features, focusing instead on stability.

Kent is confident that bcachefs is becoming very stable. He even plans to remove the "experimental" label in Linux 6.18, which is great news.

Many users are really looking forward to this! For example, Malte Schröder shared that he’s already converted his systems to bcachefs and is eager to run it on mainline Linux.

Similarly, Martin Steigerwald and Gerhard Wiesinger have confirmed its stability in their own tests, even after simulating data corruption.

Gerhard even highlighted that bcachefs could recover from corruption where btrfs could not in his tests.

Unfortunately Bcachefs is not Integrated into Kernel 6.17

The discussions around bcachefs have become quite intense in the kernel mailing list. You might be wondering, with all the positive buzz, why it isn't listed as a newly merged part of 6.17-rc1?

Well, despite the technical praise and Kent Overstreet's focus on stability and bug fixes for this release, bcachefs was not merged into the Linux 6.17 kernel.

Users like Malte Schröder even expressed their disappointment, really hoping to run bcachefs with mainline Linux again.

The main reason for this decision wasn't about the code itself, which many view as high quality. Instead, it came down to interpersonal issues and community trust.

Developers like Carl E. Thompson plainly stated that he no longer trusts Kent Overstreet "as a person enough" to use bcachefs, basing this on his own experiences and Kent's words.

Other key developers like Josef Bacik have called Kent's behaviour "toxic" and "unacceptable", noting it makes their jobs harder and leads many to filter his emails.

Kent Overstreet, on the other hand, emphasised that his main priority is ensuring bcachefs works well for its users and fixing bugs.

He felt that his efforts to get fixes out were sometimes misunderstood. He also said that the community should prioritize "doing right by users" by discussing the technical arguments openly, especially when considering a significant decision like potentially removing bcachefs from the kernel.

Kent believes the "kernel engineering processes and standards" are relevant to this discussion. He even offered to stop criticising other filesystems if it meant being able to get his work done.

Despite Kent's continuous efforts and explanations, Bcachefs was not merged into the Linux kernel tree and is now an external kernel module.

I don't know if it is ever going to be merged again into the mainline Kernel tree, but I am skeptical.

Many Notable Improvements Coming Your Way!

Even though Bcachefs is moving out-of-tree, Kent and his team continue making big improvements.

The 6.17 merge window, before the "externally maintained" status became official, saw many bugfixes and user-visible enhancements. These will now come to you via the new DKMS modules:

• Faster File Deletion: A major performance bug, which slowed down deleting many files, is now fixed.
• Better Cache Insights: The "io_read_nopromote" counter now has sub-counters. You can see these with bcachefs fs top and understand why reads might not use the cache.
• Smarter Data Promotion: Congestion tracking is now less aggressive. This means the system makes better decisions about promoting data to faster storage.
• Metadata Performance Boost: Metadata writes no longer get slowed down by writeback throttling.
• Flexible Nocow Writes: You can now rebalance "nocow" (no copy-on-write) data using options like background_target and background_compression.
• Recovery Progress Bars: Nearly all recovery passes now show clear progress indicators, so you know what is happening.
• Enhanced Repair Capabilities: Repair processes can now reconstruct missing files (inodes) even if they only find data contents, not just when the inode's internal structure is damaged.
• Improved Debugging: Btree node tracepoints now offer more detailed information for diagnostics.
• Resource Handling: Much of the code now uses a more modern resource handling system, making the codebase cleaner and more robust.

Bcachefs Needs Your Help!

Kent is actively looking for help! He encourages users to test the new DKMS packages on different distributions.

If you have experience with packaging, you can step into the bcachefs development and offer help in any way possible.

He promises more time to focus on distribution and packaging issues once rebalance_v2 and other critical tasks are complete.

Let us hope that Bcachefs continues to be a very reliable and powerful filesystem. Its community is growing, and we hope that trend continues.

Resource:

• Upcoming changes for bcachefs; notes for users distributions