Re: [PATCH] A request to reserve a "tree id" field on ext[34] inodes

[Pavel Emelyanov <xemul@openvz.org>]

Jan Kara wrote:
>   Hi,
> 
>> We have a proposal to implement a 2-level disk quota on ext3 and ext4.
>>
>> In two words - the aim is to have directories on ext3/4 partitions
>> which are limited by its disk usage and the number of inodes. Further
>> the plan is to allow configuring uid and gid quotas within them.
>   If I understand it right, this is something like XFS's project quota,
> right? 

Not exactly. XFS tree quota actually replaces gid one. My proposal is
to add the 3rd id.

> Note that such thing has implications such as you have to forbid
> hardlinks between different "quota trees", otherwise it just won't fly...

Yes, I know it. We know other things we'll have to disable, but this is
OK to live without them.

> Also by 2-level, you mean it won't be possible to nest such subtrees?

As I see it - nesting can be done on top of it. I mean - once we have
a tree id of an inode and if we say "id A is a sub-id of id B" we're done.

As far as containers are concerned - we'll have to map container id to
quota tree id, since changing a container id is fast and simple, but
it's not so for tree id. That said, this treeid is just a way do distinguish
inodes from one sub-tree from the others.

> I.e. have a quota on directories a/, b/, a/b, a/c?
> 
>> The main usage of this is containers. When two or more of them are
>> located on one disk their roots will be marked with a unique tree id
>> and thus the disk consumption of each container will be limited. While
>> achieving this goal having an id of what tree an inode belongs to is
>> a key requirement.
>>
>> So first we would like to ask to reserve a place on ext3 and ext4 inodes
>> for that ID.
>   Do you really need to store tree ID on disk? I'd think that it should
> be enough to keep some id / pointer in memory and initialize it when we
> load inode into memory (from an id / pointer of parent directory). Then
> it would be enough to store a fact that some directory is a root of
> "quota tree" somewhere - either in extended attributes, as a flag in
> the inode, or together with quota data.

We can't do it inside ext4_nfs_get_inode unfortunately :(

> 								Honza