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After executing:

sudo mount -o subvolid=0 /dev/sdc3 /mnt/temp

mount shows this:

/dev/sdc3 on /mnt/temp type btrfs (rw,relatime,ssd,space_cache,subvolid=5,subvol=/)

Why subvolid is 5 despite I specified 0 ? I don't have any subvolume with this ID. What does it mean?

1

actually from the official BTRFS wiki, it's written that the toplevel subvolid is 5. The quote :

The top-level subvolume (ID5) (which one can think of as the root of the filesystem) can be mounted, and the full filesystem structure will be seen at the mount point; alternatively any other subvolume can be mounted (with the mount options subvol or subvolid, for example subvol=subvol_a) and only anything below that subvolume (in the above example the subvolume subvol_b, it's contents, and file file_4) will be visible at the mount point

So you should either mount the filesystem without any subvol or subvolid option like this :

sudo mount /dev/sdc3 /mnt/temp

or explicitly specify the subvol of the top-level, which is 5 (not 0).

sudo mount -o subvolid=5 /dev/sdc3 /mnt/temp

In both cases you will end up with the top-level subvolume mounted.

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