Greg Freemyer <greg.freemyer@xxxxxxxxx> writes: > On Mon, Feb 8, 2010 at 8:28 AM, Dmitry Monakhov <dmonakhov@xxxxxxxxxx> wrote: >> * Abstract >> A subtree of a directory tree T is a tree consisting of a directory >> (the subtree root) in T and all of its descendants in T. >> >> Subtree feature allows to create an isolated (from user point of view) >> trees. >> >> Subtree assumptions: >> (1) Each inode has subtree id. This id is persistently stored inside >> inode (xattr, usually inside ibody) >> (2) Subtree id is inherent from parent directory >> (3) Inode can not belongs to different subtree >> Otherwise changes in one subtree result in changes in other subtree >> which contradict to isolation criteria. >> >> This feature is similar to project-id in XFS. One may assign some id to >> a subtree. Each entry from the subtree may be accounted in directory >> subtree quota. Will appear in later patches. >> >> * Disk layout >> Subtree id is stored on disk inside xattr usually inside ibody. >> Xattr is used only as a data storage, It has not user visiable xattr >> interface. >> >> Signed-off-by: Dmitry Monakhov <dmonakhov@xxxxxxxxxx> > > Dmitry, > > I think the idea of subtrees is useful, but I'm curious about other > use cases than just quota. > > At first glance you are attempting to create a generic subtree > functionality for ext4, but criteria 3) above says a inode can only be > in one subtree at a time. Theoretically this is possible, but this dramatically complicate things Just think about this. If inode belongs to different subtrees then it must have several tree-dquota objects attached to it. This means that quota require great quota redesign. Obviously i don't know any use case for this feature. do you know any? IMHO isolated subtrees has well defined use-cases. Wat's why two independent teams (xfs-team and openvz) implemented this feature in semantically identical way. > > Thus if quota utilizes subtrees and another future feature were to use > subtrees and the layout of the subtree details were not identical, > they would collide. Thus with the current patch you can only have one > subtree dependent feature at a time for a given filesystem. > > It seems you need something along the lines of a subtree name space > etc. in order to allow orthogonal service users to create orthogonal > subtrees. > > Also, I can envision use cases where you have subtrees within subtrees. > > Envision a projects folder that forms one subtree, but one specific > project within that folder needs to be in its own subtree. If I read > your patch description right, that is not allowed because only > directories of subtree 0 are allowed to contain diverging subtrees. Default tree (ID == 0) is just analog of common space. where other subrees exist. It is used for subtree manipulation. > > ie. A directory of subtree 0 can contain a directory of subtrees 1 > and 2, but a directory of subtree 10 is not allowed to contain > subtrees 11 and 12. > > Just food for thought. > > Greg -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
