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

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Date:	Tue, 09 Feb 2010 17:48:10 +0300
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Greg Freemyer <greg.freemyer@...il.com>
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

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.
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