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

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Date:	Tue, 9 Feb 2010 09:52:37 -0500
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>,
	OHSM-DEV <ohsm-devel@...ts.sourceforge.net>
Subject: Re: [PATCH 08/11] ext4: introduce subtree logic

On Tue, Feb 9, 2010 at 9:48 AM, Dmitry Monakhov <dmonakhov@...nvz.org> wrote:
> Greg Freemyer <greg.freemyer@...il.com> writes:
>
>> On Tue, Feb 9, 2010 at 8:53 AM, Dmitry Monakhov <dmonakhov@...nvz.org> wrote:
>>> Greg Freemyer <greg.freemyer@...il.com> writes:
>>>
>>>> On Tue, Feb 9, 2010 at 5:06 AM, Dmitry Monakhov <dmonakhov@...nvz.org> wrote:
>>>>> Greg Freemyer <greg.freemyer@...il.com> writes:
>>>>>
>>>>>> On Mon, Feb 8, 2010 at 8:28 AM, Dmitry Monakhov <dmonakhov@...nvz.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@...nvz.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 */
>>> }
>>
>> Conceptually agreed.
>>
>> But you'd still be limited to one subtree_entry per inode with that right?
>>
>> Another option would be more like
>>
>> #define EXT4_SUBTREE_MAX_SERVICE_GROUPS 1
>> #define EXT4_SUBTREE_MAX_SERVICE_GROUP_QUOTA 0
>>
>> // OHSM would patch the above when its ready to submit
>>
>> subtree_entry[EXT4_SUBTREE_MAX_SERVICE_GROUPS]
>> {
>>        __le16 sbe_flags /* entry flags */
>>        __le32 sbe_id    /* subtree id */
>> }
>>
>> (Clearly that won't actually work as is, but you get the idea.  Each
>> service group gets its own flags and id.)
>>
>> Another option would be separate xattr entries for each subtree group.
>>
>> Regardless, it looks very doable and its just a matter of figuring out
>> the best way to support multiple subtree service groups.
> Off course i mean that. In fact i'm plan to pack entries in to one
> xattr data block because each xattr has significant space overhead.
>

Sounds great.  Does xfs support multiple service groups with its
current offering?

Greg
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