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. -- Hugo Mills | O tempura! O moresushi! hugo@... carfax.org.uk | http://carfax.org.uk/ | PGP: E2AB1DE4 |
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