Re: [PATCH 08/11] ext4: introduce subtree logic

[Dmitry Monakhov <dmonakhov@openvz.org>]

Greg Freemyer <greg.freemyer@gmail.com> writes:

> 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.
>
> 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
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