On Fri, May 27, 2011 at 09:47:33AM +0100, Stephane Chazelas wrote: > 2011-05-27 10:21:03 +0200, Andreas Philipp: > [...] > > > What do those top-level IDs mean by the way? > > The top-level ID associated with a subvolume is NOT the ID of this > > particular subvolume but of the subvolume containing it. Since the > > "root/initial" (sub-)volume has always ID 0, the subvolumes of "depth" > > 1 will all have top-level ID set to 0. You need those top-level IDs to > > correctly mount a specific subvolume by name. > > > > # mount /dev/dummy -o subvol=<subvolume>,subvolrootid=<top-level ID> > > /mountpoint > > > > Of course, you do need them, if you specify the subvolume to mount by > > its ID. > [...] > > Thanks Andreas for pointing that subvolrootid (might be worth > adding it to > https://btrfs.wiki.kernel.org/index.php/Getting_started#Mount_Options > BTW). > > In my case, on a freshly made btrfs file system, subvolumes have > top-level 5. (and neither volume with id 0 or 5 appear in the > btrfs sub list). > > All the top-levels are 5, and I don't even know how to create a > subvolume with a different top-level there, so I wonder how that > subvol that I had created with
Actually, top-level subvolume ID=0 is a fiction. Internally, each subvolume is a separate FS tree (an FS tree in btrfs is a btree containing all of the inode and directory information for some subvolume). These trees are all referred to by a tree called the "root tree", which indexes all of the btrees in the filesystem. The root tree has a unique reference ID for each tree that it points to: most of the trees (extent tree, device tree, etc) have fixed and well-known IDs smaller than 256. The FS tree for the top-level subvolume -- the one that doesn't show up on a subvolume list -- always has ID 5. Hence the containing subvolume for most of your subvolumes is 5. The FS trees for the non-top-level subvolumes have IDs starting at 256 and increasing monotonically. Internally, there's a bit of a fiddle in the API, where a request for a subvolume ID of 0 is (sometimes) translated to an ID of 5. It's not always done, I think, and those cases where a subvol ID of 0 doesn't get you the top-level subvolume should be treated as bugs. That's all rather dense, and probably too much information. Hope it's helpful, though. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- A linked list is still a binary tree. Just a very unbalanced --- one. -- dragon
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