linux-ext4 - Re: [PATCH] A request to reserve a "tree id" field on ext[34] inodes

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Message-id: <87k4xosvb4.fsf@openvz.org>
Date:	Wed, 18 Nov 2009 00:19:59 +0300
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Pavel Emelyanov <xemul@...nvz.org>,
	Andreas Dilger <adilger@....com>,
	Theodore Ts'o <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] A request to reserve a "tree id" field on ext[34] inodes

Jan Kara <jack@...e.cz> writes:

>> 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?
>> > 
I've post fs assumptions to Andreas's replay
>> >> 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 :(
Also we will have problems with orphan list cleanup on unclean umount.
>   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
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