this post was submitted on 31 May 2026
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There is no "normal" amount of fragmentation on modern filesystems that do things like CoW. That's kind of the point.
If you're reading and writing large files with a consistent amount of I/O, you're going to have a higher amount of fragmentation because of the nature of CoW. This is by design. This doesn't mean anything is wrong with the filesystem, just that peak performance soon after writing is not achieved. Btrfs ~~and ZFS~~ do online defrag and deferred scheduling of tasks for it to allow for EVENTUAL consistency as far as contiguous block forms go. The more free space you have, the sooner it will become cleaner.
Based on my research, this has to be requested by the user/OS. I don't think Openmediavault (Debian derivative) enables auto defrag out of the box. If I set this up manually, I definitely didn't enable it. You can run a defrag command online (while the system is running) or set the mount to do it automatically.
It should be a default, but I can see why it would be disabled for SSDs to prevent using cycles unnecessarily. If you're using HDDs, check and see if it's enabled.
Either way, unless you're REALLY needing some minor performance improvements out of your disks, it shouldn't make a huge difference.