linux-ext4 - Re: Inline data with 128-byte inodes?

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Date:   Thu, 23 Apr 2020 13:56:34 +0200
From:   Jan Kara <jack@...e.cz>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     Andreas Dilger <adilger@...ger.ca>, Jan Kara <jack@...e.cz>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: Inline data with 128-byte inodes?

On Wed 22-04-20 17:40:33, Josh Triplett wrote:
> On Wed, Apr 22, 2020 at 02:15:28PM -0600, Andreas Dilger wrote:
> > On Apr 22, 2020, at 10:00 AM, Jan Kara <jack@...e.cz> wrote:
> > > On Tue 14-04-20 00:02:07, Josh Triplett wrote:
> > >> Is there a fundamental reason that ext4 *can't* or *shouldn't* support
> > >> inline data with 128-byte inodes?
> > > 
> > > Well, where would we put it on disk? ext4 on-disk inode fills 128-bytes
> > > with 'osd2' union...
> > 
> > There are 60 bytes in the "i_block" field that can be used by inline_data.
> 
> Exactly. But the Linux ext4 implementation doesn't accept inline data
> unless the system.data xattr exists, even if the file's data fits in 60
> bytes (in which case system.data must exist and have 0 length).

I see now I understand what you meant. Thanks for explanation.

> > Maybe there is a bigger win for small directories avoiding 4KB leaf blocks?
> >
> > That said, I'd be happy to see some numbers to show this is a win, and
> > I'm definitely not _against_ allowing this to work if there is a use for it.
> 
> Some statistics, for ext4 with 4k blocks and 128-byte inodes, if 60-byte
> inline data worked with 128-byte inodes:
> 
> A filesystem containing the source code of the Linux kernel would
> save about 1508 disk blocks, or around 6032k.
> 
> A filesystem containing only my /etc directory would save about 650
> blocks, or 2600k, a substantial fraction of the entire directory (which
> takes up 9004k total without inline data).

I guess few people care about a few megabytes these days... For really
space sensitive applications, people don't pick ext4 as a base filesystem
I'd guess (I'd expect squashfs, erofs, or ubifs if you need write access).
So the benefit is relatively small, the question about the cost is - how
complicated it gets to support inline data without xattrs?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR