Hugo Mills <hugo <at> carfax.org.uk> writes: > > On Fri, Apr 17, 2015 at 06:24:05AM +0000, sri wrote: > > Hi, > > I have below queries. Could somebody help me in understanding. > > > > 1) > > As per my understanding btrfs file system uses one chunk tree and one > > extent tree for entire btrfs disk allocation. > > > > Is this correct? > > Yes. > > > In, some article i read that future there will be more chunk tree/ extent > > tree for single btrfs. Is this true. > > I recall, many moons ago, Chris saying that there probably wouldn't > be. > > > If yes, I would like to know why more than one chunk / extent tree is > > required to represent one btrfs file system. > > I think the original idea was that it would reduce lock contention > on the tree root. > > > 2) > > > > Also I would like to know for a subvolume / snapshot , is there a > > provision to ask btrfs , represent all blocks belongs to that > > subvolume/snapshot should handle with a separate chunk tree and extent > > tree? > > No. > > > I am looking for a way to traverse a subvolume preferably a snapshot and > > identify all disk blocks (extents) allocated for that particular subvolume > > / snapshot. > > Do you mean allocated to any file in the subvolume, or do you mean > *exclusively* allocated to that subvolume and not shared with any > other? > > The former is easy -- just walk the file tree, and read the extents > for each file. The latter is harder, because you have to look for > extents that are not shared, and extents that are only shared within > the current subvolume (think reflink copies within a subvol). I think > you can do that by counting backrefs, but there may be big race > conditions involved on a filesystem that's being written to (because > the backrefs aren't created immediately, but delayed for performance > reasons). > > Note that if all you want is the count of those blocks (rather than > the block numbers themselves), then it's already been done with > qgroups, and you don't need to write any btrfs code at all. > > What exactly are you going to be doing with this information? > > Hugo. >
I am trying a way to get all files and folders of a snapshot volume without making file system level calls (fopen etc..) I want to write code to understand the corresponding snapshot btree and used related chunk tree and extent tree, and find out for each file (inode) all extent blocks. If I want to backup, I will use above method to traverse snapshot subvolume at disk level and copy all blocks of files/directories. Thank you sri -- 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
