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

[Dmitry Monakhov <dmonakhov@openvz.org>]

Andreas Dilger <adilger@sun.com> writes:

> On 2010-02-08, at 06:28, Dmitry Monakhov wrote:
>>  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.
>
>
> Presumably this would need e2fsck support, in order to ensure that
> inodes in a given directory have the same subtree ID as the parent
> directory.
Yes i'm working on this, I've already prepared ioctl support
for chattr/lsattr.
> Also, how are the subtree IDs generated, to ensure that they are
> unique between subtrees?
This is user responsibility. There is possible situation where 
set of subtrees may has same ID. The main idea it that each of
such subtrees is a *full* subtree (it includes all nodes starting
from the subtree root)
About isolation: currently there is a discussion about this feature
some people state that subtrees may exist without a isolation
requirement. Let's wait where it goes.
>
> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.