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

[Dmitry Monakhov <dmonakhov@openvz.org>]

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

> On Tue, Feb 9, 2010 at 5:06 AM, Dmitry Monakhov <dmonakhov@openvz.org> wrote:
>> 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?
>
> Maybe we're talking about different things.  I working with the OHSM
> project <http://ohsm.sourceforge.net/>
>
> We haven't submitted any patches yet, but for one of our features we
> have something fairly close to your subtree patch.  If we were to
> leverage your patch and drop that part of ours we would be in the
> unhappy situation that quota and ohsm could not both be enabled on the
> same filesystem because the ohsm subtree geography is not likely to be
> consistent with the quota subtree geography.
>
> If we call quota and ohsm services then my desire would be to see your
> subtree patches support orthoganal subtree groups.  One group per
> service.
>
> I haven't looked into the actual implementation, but from an API
> perspective it is just a matter of adding a service parameter to the
> various calls.  For a given subtree service group, a given inode could
> only be part of one subtree, but a single inode could participate in
> multiple subtree service groups.
Ok now i think i understand what you are talking about.
one subtree <=> one service is the main rule i'm standing.
If you wan to support several services, no problem
I can easily extend xattr to support different services
something like this
subtree_entry
{
        __le16 sbe_flags /* entry flags */
        __le16 sbe_type  /* service type */
        __le32 sbe_id    /* subtree id */
}
/* subtree entry flags
enum {
     SUBTREE_FL_VALID           = 1, /* entry contains valid data */
     SUBTREE_FL_INHERENT        = 2, /* inherent id from parent on
                                                            create *
     SUBTREE_FL_ISOLATE         = 4  /* Isolated subtree */
    /* and soon */
};

>
> Greg
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