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

[Jan Kara <jack@suse.cz>]

> 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.
  Yeah, OK, but it's quite similar :)

> > 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.
  But for implementation, it's kind of important whether there is going
to be just one "tree" limitation for each inode, or arbitrary number of
them...

> > 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 :(
  Right, that's nasty. OK, but as Andreas suggested, extended attributes
are more flexible for this - most notably every fs supporting them would
be able to support your tree quota extension.

								Honza
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs