Question: Why is it recommended to mount `subvolid=5` when backing up `/`?

[camoz]

The README states:

Mounting subvolid=5 is recommended (mandatory for btrbk < v0.32.0) if you want to backup your root filesystem /.

I assume "root filesystem /" here refers to the OS root, not the top-level subvolume.

Why is it recommended to do that? I.e., why is it recommended against doing:

snapshot_dir /mnt/snapshots
subvolume /

[ATLief]

@camoz I'm not affiliated with the project (yet), but it might be to generate more human-readable snapshot "names" (the string in the snapshot filename). The filename of your sub-volume can (and probably will be) descriptive of the content, resulting in its snapshots having that property too. I would also imagine that btrbk doesn't have as easy access to the sub-volume name when all it's told is "/".

You may not be aware of btrfs's ability to mount sub-volumes as if they were the root of the filesystem, which makes using a sub-volume for the main content very easy to manage.

[digint]

It's less of a technical restriction (btrfs should be able to cope with that), here's some reasons that come into my mind:

• If you do that, you are making snapshots into something you want to backup. While this works in btrfs, it still seems very wrong to me.
• On send/receive, nested subvolumes will appear as directories on the receiving side. In your example, there will be directories like /mnt/snapshots/rootfs.20220101 on the receiving side. In my opinion this is wrong (have not checked, I believe this is still the case and will ever be), but it's how btrfs does it.
• Unsure what the behavior of onchange would be: creating a snapshot within the root filesystem should (in my logic) alter the root filesystem, making onchange obsolete. Somehow it is not the case, somehow creating a snapshot does not increase the generation (transid). Needs more testing...

I do not want to give an exhaustive explanation in this part of the README. But you are right, if I'm recommending something I should probably say why somewhere. Gladly taking suggestions, will think of it again later (and probably do some tests on how btrfs behaves exactly).

[camoz]

Ah, I think I have not correctly described what I mean...

I was talking about a btrfs layout where /mnt/snapshots is not a nested subvolume below /, like this:

subvolume                   mountpoint
----------------------------------------
id=5                        /mnt/btrfsvol
+-@snapshots                /mnt/snapshots
+-@                         /
+-@home                     /home

With a setup like this, is there still a (great) difference between using subvolid=5 and using the (non-nested) /mnt/snapshots for backing up the root subvolume? I assume no...

[skug67]

UPDATE: Had installed via Debian (on bullseye) so was running an older version. Updated to 0.32 (using the bookworm repository) and was able torun using my subvolume/mount setup. Apologies for the uninformed post below and thanks for the great project.

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If I understand the point @camoz is trying to make, then I agree with it. I similarly have a subvolume "@" that is mounted as my root filesystem and a separate subvolume "@snapshots" mounted elsewhere (in my case it's not /mnt/snapshots, but that hardly matters). It seems like it would be much more convenient to be able to invoke btrbk by setting those two locations as the subvolume and snapshot_dir parameters (respectively) in btrbk.conf rather than needing to separately mount subvolid=5 someplace.