linux-ext4 - Re: [PATCH v2 19/28] mke2fs: add inline

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Date:	Tue, 3 Dec 2013 22:02:24 -0800
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
	Zheng Liu <wenqing.lz@...bao.com>, Tao Ma <tm@....ma>
Subject: Re: [PATCH v2 19/28] mke2fs: add inline_data support in mke2fs

On Wed, Dec 04, 2013 at 01:50:52PM +0800, Zheng Liu wrote:
> [Cc Tao to get some comments]
> 
> On Tue, Dec 03, 2013 at 09:26:08PM -0800, Darrick J. Wong wrote:
> > On Wed, Dec 04, 2013 at 01:21:50PM +0800, Zheng Liu wrote:
> > > On Tue, Dec 03, 2013 at 08:08:19PM -0800, Darrick J. Wong wrote:
> > > > On Wed, Dec 04, 2013 at 11:27:57AM +0800, Zheng Liu wrote:
> > > > > On Tue, Dec 03, 2013 at 02:30:26PM -0800, Darrick J. Wong wrote:
> > > > > [...]
> > > > > > > +		 * notify users that inline data will never be useful.
> > > > > > > +		 */
> > > > > > > +		if ((fs_param.s_feature_incompat &
> > > > > > > +		     EXT4_FEATURE_INCOMPAT_INLINE_DATA) &&
> > > > > > > +		    inode_size == EXT2_GOOD_OLD_INODE_SIZE) {
> > > > > > 
> > > > > > Perhaps I'm missing something here, but why is it impossible to use i_blocks
> > > > > > for inline data even if there's no space for EAs?
> > > > > 
> > > > > If I understand correctly, on kernel side, we determine an inode has
> > > > > inline data according to whether we have 'system.data' xattr entry on
> > > > > inode extended attribute space.  If an inode doesn't have enough space
> > > > > to store an entry with 'system.data', we just think this inode doesn't
> > > > > has inline data.  So that is why I add this sanity check.
> > > > 
> > > > Ok.  I was curious.  Small inode => no inline data seems like an unfortunate
> > > > restriction to me, but oh well, it's your feature.  I don't plan to go back to
> > > > 128 byte inodes ever. :)
> > > > 
> > > > Also, we could store four more bytes if we created a new e_name_index value (5?
> > > > 9?) to represent "system.data".  Any thoughts about that?
> > > 
> > > Sorry, I don't get your point.  Do you want to create a new e_name_index?
> > > Any reason lets you want to do this?
> > 
> > Yep, that's exactly what I propose to do, so we can cram four more bytes into
> > the inline data.
> 
> Agree.  I believe it is fine.  But I am wondering if it will break the
> file system that inline data has been enabled.

There's a fair amount of work needed for fs/ext4/inline.c.  My e2fsprogs thing
should handle it fine, though I think the inlinedata_max_size function
somewhere in your patchset might also be broken.

I suspect a lot depends on how widely deployed inlinedata is inside Taobao, or
anyone else who's actually running it right now.

--D
> 
>                                                 - Zheng
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