> As I am writing some documentation abount creating snapshots: > Is there a generic name for both volume and subvolume root?
Yes, it is from the UNIX side 'root directory' and from the Btrfs side 'subvolume'. Like some other things Btrfs, its terminology is often inconsistent, but "volume" *usually* means "the set of devices [and contained root directories] with the same Btrfs 'fsid'". I think that the top-level subvolume should not be called the "volume": while there is no reason why a UNIX-like filesystem should be limited to a single block-device, one of the fundamental properties of UNIX-like filesystems is that hard-links are only possible (if at all possible) within a filesystem, and that 'statfs' returns a different "device id" per filesystem. Therefore a Btrfs volume is not properly a filesystem, but potentially a filesystem forest, as it may contain multiple filesystems each with its own root directory. > Is there a simple name for directories I can snapshot? You can only snapshot *root directories*, of which in Btrfs there are two types: subvolumes (an unfortunate name perhaps) or snapshots. In UNIX-like OSes every filesystem has a "root directory" and some filesystem types like Btrfs, NILFS2, and potentially JFS can have more than one, and some can even mount more than one simultaneously. The root directory mounted as '/' is called the "system root directory". When unmounted all filesystem root directories have no names, just an inode number. Conceivably the root inode of a UNIX-like filesystem could be an inode of any type, but I have never seen a recent UNIX-like OS able to mount anything other than a directory-type root inode (Plan 9 is not a UNIX-like OS :->). As someone else observed, the word "root" is overloaded in UNIX-like OS discourse, like the word "filesystem", and that's unfortunate but can always be resolved verbosely by using the appropriate qualifier like "root directory", "system root directory", "'root' user", "uid 0 capabilities", etc. -- 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
