Greg Freemyer <greg.freemyer@gmail.com> writes:
quoted text > On Mon, Feb 8, 2010 at 8:28 AM, Dmitry Monakhov <dmonakhov@openvz.org> 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@openvz.org>
>
> 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.
quoted text >
> 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.
quoted text >
> 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
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