Change btrfs default subvolume in order to delete snapshot

Question

I have used snapper with Fedora 26 to create snapshots of my btrfs system, and I am trying to start fresh (i.e. remove snapper and anything it has done) but I have one snapshot I cannot remove as it appears to be my default subvolume (I don't remember why it would have done this, perhaps I tried a rollback).

# snapper delete 535
Deleting snapshot failed.
# btrfs subvolume get-default /
ID 1630 gen 112224 top level 275 path .snapshots/535/snapshot
# btrfs subvolume show /
/
    Name:                   root
    UUID:                   544f9215-4537-fe4b-b1b5-4da21d9abb9c
    Parent UUID:            -
    Received UUID:          -
    Creation time:          2017-07-19 21:28:42 +0100
    Subvolume ID:           257
    Generation:             112591
    Gen at creation:        8
    Parent ID:              5
    Top level ID:           5
    Flags:                  -
    Snapshot(s):

I am not certain if what I am asking here makes sense, but I want to change my default subvolume back to the parent (;master;root, not sure the right word) volume I had when I originally set up the system at the top-level of the file system, i.e. /, before the default change.

# btrfs subvolume list / -a -p -t
ID      gen     parent  top level       path
--      ---     ------  ---------       ----
257     112604  5       5               <FS_TREE>/root
258     112604  5       5               <FS_TREE>/home
263     112214  257     257             root/var/lib/machines
275     112593  257     257             root/.snapshots
276     112577  258     258             <FS_TREE>/home/.snapshots
1630    112224  275     275             <FS_TREE>/root/.snapshots/535/snapshot
1639    112604  257     257             root/var/log
1641    111548  257     257             root/mnt/virtualbox

Can I run a btrfs subvolume set-default to one of these to achieve this. I'm not certain on the meaning of /... as I was guessing I should change the default to the root with ID 257, but this threw me off.

The files inside this snapshot are now outdated compared to those in / and so I am not looking to overwrite / with the contents of the snapshot.

Answer 1

With mounting a BTRFS filesystem, it's possible to specify which subvolume to mount, or not:

A Btrfs filesystem has a default subvolume, which is initially set to be the top-level subvolume [who's ID is always 5] and which is mounted if no subvol or subvolid option is specified. - https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Snapshots

You can override the default subvolume by using the subvol option when mounting the filesystem:

mount /dev/sda / -o subvol=root/.snapshots/535/snapshot

In your case, subvolume 275 was changed to be the default when you performed a rollback.

rollback [options] [number]

Creates two new snapshots and sets the default subvolume. Per default the system boots from the default subvolume of the root filesystem. - http://snapper.io/manpages/snapper.html

What the Snapper map page is saying here implicitly is that it expects the mount point for your root filesystem to not specify a subvolume. Otherwise the rollback command would have no effect since it would be overwritten by the subvol option.

Your original default subvolume was the top-level (ID 5). You can certainly use btrfs subvolume set-default to make it the default again, but... perhaps you shouldn't.

If you've been booting from your 535 subvolume, then perhaps you've accumulated a vast number of changes over time on that subvolume. Reverting to your top-level subvolume would mean the loss of those changes; I'm using the word loss loosely here given that the changes would still exist, just in a different subvolume.