5

Trying to create a read-only snapshot of the root subvolume using

sudo btrfs sub snap -r /mnt/@ /mnt/@_20200510

leads to error message

Create a readonly snapshot of '/mnt/@' in '/mnt/@_20200510'
ERROR: cannot snapshot '/mnt/@': Text file busy

Also, snapper and other backup tools stopped working.

1 Answer 1

6

Turn off and delete the swapfile.

Linux 5.x introduced official swapfile support for Btrfs. Unfortunately, this support appears to be, ahem, incomplete. I suppose what is happening is that a subvolume which contains an open non-COW file cannot be snapshotted (snapshot? snapfleshwounded?). On some level this makes complete sense. On another, wtf. sigh.

If you want to use a swapfile, I think it should be fine to create a separate subvolume, however I have not tested this. Otherwise, you really do need to delete the swapfile before making the snapshot otherwise the file will get reflinked, stop being non-COW even though you marked it as such, and swapon won't work.

1
  • Creating a separate subvolume is described here, and works for me under Ubuntu 21.04 with 5.11 kernel.
    – rvf
    May 24, 2021 at 11:28

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