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Is scanning necessary before mounting btrfs partition?
Executive summary
Scanning is not strictly required every time before mounting a Btrfs filesystem, but kernels and user-space tooling often perform or expect a "device scan" so that multi-device filesystems are registered with the kernel; system distributions commonly run such scans at boot via initramfs hooks or a btrfs-scan service [1] [2] [3]. Tools to check or repair Btrfs (btrfs check / btrfs-check) are explicitly recommended to run on an unmounted filesystem, because running checks or repairs on a mounted filesystem can be dangerous [4] [5] [6].
1. Why people ask “is scanning necessary?” — a boot-time friction story
Many distributions add initramfs hooks or services that modprobe the btrfs module and run a device scan during boot; that action shows up as “Scanning for Btrfs filesystems” and can delay start-up or block mounts when the scan behavior or device ordering is unexpected [7] [8]. Users who never use Btrfs sometimes see the message and conclude the system is needlessly scanning; removing btrfs-tools/progs and updating initramfs eliminates those hooks and the boot-time scan [7] [8].
2. What “btrfs device scan” actually does and when it matters
btrfs device scan registers Btrfs devices with the kernel and lets automatic mounting tools wait until the scan completes; that is useful (and sometimes required) for multi-device filesystems so the kernel knows all members before mounting, and for initial ramdisks to find a raid/group device from just one device argument [1] [3]. The documentation states the scan is only one precondition for mount and normal users “may safely ignore it” in many cases — but ignoring it can make multi-device mounts fail if some devices aren’t registered [1].
3. When you can skip an explicit scan — single-device and routine cases
If your filesystem is single-device, the kernel and udev will typically expose the device and mount will succeed without a manual btrfs device scan; the Btrfs docs note device scanning is usually done automatically by a device manager and the mount option device or the device scan command are workarounds when that automatic discovery does not work [3] [9]. In other words: for ordinary single-disk setups you usually do not need to run an explicit scan before mounting [3].
4. When you should run a scan or enable the scan service
If you have a multi-device Btrfs (RAID, multiple devices) or if devices can appear late (USB sticks, removable extroot) the btrfs device scan or device= mount option is important — OpenWrt and other embedded examples put device scan before and after root mount for this reason [10]. Some distros ship a btrfs-scan service that loads the kernel module and registers Btrfs devices early; enabling that service avoids failures to mount at boot on systems using Btrfs for root [2].
5. Scanning vs. checking: don’t conflate the two
Users sometimes conflate "scan" (discover and register devices) with "check" (verify filesystem structure). btrfs device scan just discovers/ registers devices; btrfs check (btrfs-check/btrfs-progs) inspects and can repair filesystem structures and the manuals explicitly recommend unmounting before running those checks because checking or repairing a mounted fs can corrupt it [4] [5] [6]. The repair tools are described as immature enough that backups are recommended before using --repair [6].
6. Practical guidance distilled from the docs and reporting
- For single-device use or routine mounts: you generally do not need to run an explicit btrfs device scan; automatic device managers usually suffice [3] [1].
- For multi-device Btrfs, removable devices, or boot-time failures to find members: run btrfs device scan (or add device=/dev/… to mount options) or enable the distro’s btrfs-scan/initramfs hook so all devices are registered before mount [1] [10] [3].
- For filesystem integrity work: use btrfs check on an unmounted filesystem; avoid repair on mounted filesystems unless documentation and circumstances explicitly permit and you have backups [4] [5] [6].
7. Limitations and unresolved questions in current reporting
Available sources do not quantify how often automatic udev/ kernel discovery fails in modern kernels, nor do they give a deterministic checklist for every distribution’s initramfs behavior — you must check your distro’s initramfs hooks, presence of btrfs-tools/progs, and whether a btrfs-scan service exists (not found in current reporting). Also, while multiple community threads report boot delays caused by scanning hooks, that reporting is experiential and distribution-specific rather than a universal rule [7] [8] [11].
If you tell me your setup (single vs multi-device, removable vs internal, distro and whether Btrfs is root), I can translate those rules into exact commands or service changes tailored to your case and cite the exact docs that apply [1] [3] [2].