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'Unbreakable' Oracle Linux 9 is a RHEL rebuild with built-in Btrfs support

Red Hat dropped the file system in 7.4 but Big Red's own kernel has kept hold

Oracle Linux 9 is out and has some interesting differences from the other Red Hat relatives.

The version was released at the end of June, marking an unusually long gap from Red Hat's announcement of RHEL 9 the month before. For comparison, the beta of AlmaLinux 9 came just three days after RHEL 9's official availability on May 17, and the final version followed within a week.

A similar delay seems to be affecting Rocky Linux as well. Nearly a month after Red Hat's announcement of RHEL 9, the Rocky Linux team tweeted that Rocky 9 was coming soon.

Oracle's release notes reveal an interesting change. Under the heading "Package Changes from the Upstream Release", the second item in the list is btrfs-progs. Given that Red Hat explicitly no longer supports Btrfs in RHEL, this is unexpected.

Oracle supplies a choice of kernels with Oracle Linux, along with documentation on how to switch between them. One is called the "Red Hat Compatibility Kernel" (RHCK) and the other the Unbreakable Enterprise Kernel (UEK), which is Oracle's modified build of the kernel. If you need strict RHEL compatibility – the target that AlmaLinux and Rocky Linux provide – then RHCK is identical to the kernel in RHEL, just as CentOS Linux used to be. UEK is something different and, among other things, includes Btrfs support.

To be clear, this isn't something that has just appeared. Oracle Linux supported Btrfs as far back as version 6 – but back then, RHEL supported it too. Btrfs was removed from RHEL version 7.4 onward, so what is interesting is that Oracle has kept it in version 8 and subsequent releases.

Given that Oracle acquired Sun and its Solaris OS, and still offers it, you might reasonably expect that if Oracle were to add an advanced file system to its Linux, it would be Solaris's ZFS, presumably in its Linux incarnation as OpenZFS. Not so: although it is possible to add OpenZFS to RHEL-family distros, it requires adding an external repository.

The CentOS Stream project also recently announced a new head of its Hyperscale Special Interest Group (SIG). Hyperscale users are those deploying CentOS Stream onto very large numbers of servers at once – think large social network companies, for instance. In this announcement too, there's mention of Btrfs:

The brtfs-progs package was updated to 5.16.2 on both CentOS Stream 8 and CentOS Stream 9.

As The Reg has reported before, one notable large user of Btrfs is Facebook.

Red Hat is working on a next-gen Rust-based storage management tool called Stratis. Development has been under way since 2016, and preliminary support appeared in Fedora 29 in 2018 and CentOS 8 in 2019. Connecting the dots and extrapolating a little here, The Reg FOSS desk wonders if support for Btrfs in Fedora 33 and now in CentOS Stream might have bad implications about how well Stratis is progressing. ®

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