linux-ext4 - Re: [GIT PULL] Ext3 removal, quota & udf fixes

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Message-ID: <20150902184522.GA10390@birch.djwong.org>
Date:	Wed, 2 Sep 2015 11:45:22 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [GIT PULL] Ext3 removal, quota & udf fixes

On Wed, Sep 02, 2015 at 05:52:01PM +0100, Mel Gorman wrote:
> On Mon, Aug 31, 2015 at 02:37:38PM -0700, Linus Torvalds wrote:
> > On Sun, Aug 30, 2015 at 11:19 PM, Jan Kara <jack@...e.cz> wrote:
> > >
> > > The biggest change in the pull is the removal of ext3 filesystem driver
> > > (~28k lines removed).
> > 
> > I really am not ready to just remove ext3 without a lot of good
> > arguments. There might well be people who this use ext3 as ext3, and
> > don't want to update. I want more a rationale for removal than "ext4
> > can read old ext3 filesystems".
> > 
> 
> This is not my area at all but as Jan said he was out on vacation and
> offline so there is no chance for him to adjust the tree before the window
> closes. I'm going going to try and guess what justifications he might have
> used if he was online.
> 
> 1. Backwards compatibility -- other knowledgeable people, particularly
>    Ted, already pointed out that backwards compatibility is guaranteed.
>    I know SLE is using the ext4 driver for ext3 filesystems and AFAIK,
>    there has been no bugs related to distro upgrades that failed to mount
>    an ext3 filesystem with the ext4 driver. As other distributions made
>    a similar decision and there is a lack of bug reports, there is some
>    evidence that the guarantee is adhered to
> 
> 
> 2. ext4 driver performance -- when SLE considered switching to the ext4
>    driver, I successfully checked that the ext4 driver matched or exceeded
>    the performance of the ext3 driver. Granted, this was limited in terms
>    of types of storage but as other distros are also using ext4 driver,
>    I'm guessing that no one found regressions. I don't have the data any
>    more but I don't recall a single instance where the ext3 driver was better
> 
> 2. ext3-specific hack removals in block and VM. The merge request stated
>    that some workarounds in the VM and block layer could be got rid of but
>    I don't have a comprehensive list. Glancing at the branch though, at
>    least one hack is removed with "block: Remove forced page bouncing under

I would be happy if the fs bounce buffering band-aid went away forever. :)

>    IO". I did not investigate deeply but it looks like cancel_dirty_page
>    is another potential candidate for going away.
> 
> 3. Missing fixes. Fixes applied to ext4 have to be manually back-ported
>    to ext3, mostly by Jan, but it's possible one will be missed and ext3
>    slowly bit rots. Ted already said this a lot better than I did so I'll
>    just repeat it
> 
> 	Both Red Hat and SuSE, as well as Debian and Ubuntu, are using
> 	ext4 with CONFIG_EXT4_USE_FOR_EXT23 for a couple of years now
> 	to support ext2 and ext3 file systems.	So with the exception of
> 	some really ancient enterprise Linux distros, and people who are
> 	manually configuring their systems, very few people are likely using
> 	ext3 code base, which means the chances that it bitrots increases.
> 	Basically, it's only been Jan's tireless work that has kept that
> 	from happening, given that all of the major distro's have been
> 	using ext4 to support ext2 and ext3 file systems.
> 
> On the flip side, there does not appear to be any good reason for
> keeping the ext3 driver around because if there ever is a case where an
> old kernel is required to mount an ext3 filesystem then it appears the
> ext4 developers would consider it a bug.

Yes, that would be a bug.

--D

> 
> -- 
> Mel Gorman
> SUSE Labs
> --
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