linux-ext4 - Re: [PATCH v1 0/9] do not use s

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Date:	Thu, 22 Mar 2012 12:05:47 +0200
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Ted Tso <tytso@....edu>,
	Ext4 Mailing List <linux-ext4@...r.kernel.org>,
	Linux FS Maling List <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Maling List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 0/9] do not use s_dirt in ext4

On Thu, 2012-03-22 at 10:53 +0100, Jan Kara wrote:
> On Tue 20-03-12 16:41:20, Artem Bityutskiy wrote:
> > This patch-set makes ext4 independent of the VFS superblock management
> > services. Namely, ext4 does not require to register the 'write_super()' VFS
> > call-back.
> > 
> > The reason of this exercises is to get rid of the 'sync_supers()' kernel thread
> > which wakes up every 5 seconds (by default) even if all superblocks are clean.
> > This is wasteful from power management POW (unnecessary wake-ups).
> > 
> > Note, I tried to optimize 'sync_supers()' instead in 2010, but Al wanted me
> > to get rid of it instead. See https://lkml.org/lkml/2010/6/6/87
> > And I think this is right because many file-systems do not need this, for
> > example btrfs does not use VFS superblock management services at all, so on a
> > btrfs-based system we currently end-up useless periodic wake-ups source.
> > 
> > Changes for other file-systems are coming later.
> > 
> > The patch-set structure.
> > 1. patches 1,2,3 are independent ext4 cleanups and I ask Ted to merge them as
> >    soon/long as they are OK. I sent them also independently in order to get
> >    early comments, but did not get so far, so re-sending.
> > 2. patch 4 exports 'dirty_writeback_interval' and it would be very useful to
> >    have it merged ASAP to simplify further work
> > 3. patch 5 is also and independent VFS clean-up
> > 4. patches 6-9 actually make ext4 independent on the 'sync_supers()' thread.
>   Artem, if you look at places where ext4 sets s_dirt you will notice they
> are rather rare events and all of them actually take care of writing
> superblock themselves (at least if my memory serves well). So ext4
> shouldn't need sync_supers() at all...

Hmm, if there is journal, then ext4 does not initialize the
'->write_super()' VFS call-back and indeed takes care of the SB itself.
Indeed. So 'sync_supers()' wakes up every 5 seconds for nothing.

However, if there is _no_ journal, the 'write_super' is initialized, and
in many places the 's_dirt' flag is set, and thus VFS services seem to
be actively used.

I do not ext4 well enough, but the SB dirtying looks a bit messy, and I
am happy to do some clean-ups, just need a bit more of direction :-)

Thanks!

-- 
Best Regards,
Artem Bityutskiy

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