On 09/30/2017 04:53 PM, Peter Grandi wrote: >> My Hypothesis, it should be the ID of the root subvolume ( or >> 5 if it is not mounted). [ ... ] > Well, a POSIX filesystem typically has a root directory, and it > can be mounted as the system root or any other point. A Btrfs > filesystem has multiple root directories, that are mounted by > default "somewhere" (a design decision that I think was unwise, > but "whatever").
I agree that the possibility to create a subvolume/snapshot everywhere could create a bit of confusion; i.e. the "mountpoint" of a subvolume after a snapshot is a very strange beast: ghigo@venice:/tmp$ btrfs sub cre sv1 Create subvolume './sv1' ghigo@venice:/tmp$ btrfs sub cre sv1/sv2 Create subvolume 'sv1/sv2' ghigo@venice:/tmp$ btrfs sub snap sv1 sn1 Create a snapshot of 'sv1' in './sn1' ghigo@venice:/tmp$ ls -lid sn1/ 256 drwxr-xr-x 1 ghigo ghigo 6 Oct 2 20:54 sn1/ ghigo@venice:/tmp$ ls -li sn1/ total 0 2 drwxr-xr-x 1 root root 0 Oct 2 20:55 sv2 ghigo@venice:/tmp$ touch sn1/sv2/foo touch: cannot touch 'sn1/sv2/foo': Permission denied ghigo@venice:/tmp$ sudo touch sn1/sv2/foo touch: cannot touch 'sn1/sv2/foo': Permission denied It seems a directory, but it is not allowed to create file inside (!) However the user is free to limit itself to create snapshot only under / (rootid==5) :-). > The subvolume containing the mountpoint directory of another > subvolume's root directory is is no way or sense its "parent", > as there is no derivation relationship; root directories are > independent of each other and their mountpoint is (or should be) > a runtime entity. This is true. However I think that "btrfs fi list" was designed to show where subvolumes (or snapshots) are, for two reasons: 1) avoid to search trough all the filesystem looking for directory having inode=256 2) when the user mounts a subvolume as root, he has not direct access to all subvolume unless he also mount / (rootid=5) on another place. "btrfs sub list" has not this limit BTW, my preferred choice is: - mount a subvolume as root; - mount the root filesystem (rootid=5) under /var/btrfs - make snapshot and/or subvolume only under / (rootid=5), handle these in /var/btrfs - if needed, mount a subvolume in the filesystem through fstab; - however I am still searching a way to handle the snapshots which fully satisfy me (putting them under /, is not the best) BR -- gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it> Key fingerprint BBF5 1610 0B64 DAC6 5F7D 17B2 0EDA 9B37 8B82 E0B5 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
