Date:	Wed, 17 Oct 2012 07:30:21 +0900
From:	Jaegeuk Kim <jaegeuk.kim@...il.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Jaegeuk Kim <jaegeuk.kim@...sung.com>,
	'Vyacheslav Dubeyko' <slava@...eyko.com>,
	viro@...iv.linux.org.uk, 'Theodore Ts'o' <tytso@....edu>,
	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	chur.lee@...sung.com, cm224.lee@...sung.com,
	jooyoung.hwang@...sung.com
Subject: Re: [PATCH 11/16] f2fs: add inode operations for special inodes

> > > > > The main reason I can see against extended attributes is that they are not stored
> > > > > very efficiently in f2fs, unless a lot of work is put into coming up with a good
> > > > > implementation. A single flags bit can trivially be added to the inode in
> > > > > comparison (if it's not there already).
> > > > 
> > > > That's a deficiency that should be corrected, then, because xattrs
> > > > are very common these days.
> > > 
> > > IMO, most file systems including f2fs have some inefficiency to store
> > > and retrieve xattrs, since they have to allocate an additional block.
> > > The only distinct problem in f2fs is that there is a cleaning overhead.
> > > So, that's the why xattr is not an efficient way in f2fs.
> > 
> > I would hope that there is a better way to encode extented attributes
> > if the current method is not efficient enough. Maybe Dave or someone
> > else who is experienced with this can make suggestions.
> > 
> > What is the "expected" size of the attributes normally?
> 
> Most attributes are small. Even "large" user attributes don't
> generally get to be more than a couple of hundred bytes, though the
> maximum size for a single xattr is 64K.
> 
> > Does it
> > make sense to put attributes for multiple files into a single block?
> 
> There are two main ways of dealing with attributes. The first is a
> tree-like structure to index and store unique xattrs, and have the
> inode siimply keep pointers to the main xattr tree. This is great
> for keeping space down when there are lots of identical xattrs, but
> is a serialisation point for modification an modification can be
> complex (i.e. shared entries in the tree need COW semantics.) This
> is the approach ZFS takes, IIRC, and is the most space efficient way
> of dealing with xattrs. It's not the most performance efficient way,
> however, and the reference counting means frequent tree rewrites.
> 
> The second is the XFS/ext4 approach, where xattrs are stored in a
> per-inode tree, with no sharing. The attribute tree holds the
> attributes in it's leaves, and the tree grows and shrinks as you add
> or remove xattrs. There are optimisations on top of this - e.g. for
> XFS if the xattrs fit in the spare space in the inode, they are
> packed into the inode ("shortform") and don't require an external
> block. IIUC, there are patches to implement this optimisation for
> ext4 floating around at the moment. This is a worthwhile
> optimisation, because with a 512 byte inode size on XFS there is
> enough spare space (roughly 380 bytes) for most systems to store all
> their xattrs in the inode itself. XFS also has "remote attr storage"
> for large xattrs (i.e. larger than a leaf block), where the tree
> just keeps a pointer to an external extent that holds the xattr.

Thank you for great explanation. :)

> 
> IIRC, fs2fs uses 4k inodes, so IMO per-inode xattr tress with
> internal storage before spilling to an external block is probably
> the best approach to take...

Yes, indeed this is the best approach to f2fs's xattr.
Apart from giving fs hints, it is worth enough to optimize later.

> 
> > > OTOH, I think xattr itself is for users, not for communicating
> > > between file system and users.
> > 
> > No, you are mistaken in that point, as Dave explained.
> 
> e.g. selinux, IMA, ACLs, capabilities, etc all communicate
> information that the kernel uses for access control. That's why
> xattrs have different namespaces like "system", "security" and
> "user". Only user attributes are truly for user data
> - the rest are for communicating information to the kernel....
> 

I agree that "system" is used by kernel.
How about the file system view?
Would you explain what file systems retrieve xattrs and use
them with their own purpose?
Sorry, I'm not familiar with xattrs in depth.

Unfortunately, "system" is not implemented in f2fs yet. :(

> A file usage policy xattr would definitely exist under the "system"
> namespace - it's not a user xattr at all.
> 
> Cheers,
> 
> Dave.

-- 
Jaegeuk Kim
Samsung

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