linux-kernel - Re: [PATCHv4 3/5] ext4: remove unnecessary superblock dirtying

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Date:	Tue, 10 Jul 2012 15:17:29 +0300
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Theodore Tso <tytso@....edu>,
	Linux FS Maling List <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Maling List <linux-kernel@...r.kernel.org>,
	Ext4 Mailing List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCHv4 3/5] ext4: remove unnecessary superblock dirtying

On Wed, 2012-07-04 at 15:11 +0200, Jan Kara wrote:
> On Wed 04-07-12 15:21:52, Artem Bityutskiy wrote:
> > From: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> > 
> > This patch changes the '__ext4_handle_dirty_super()' function which is used
> > by ext4 to update the superblock via the journal in the following cases:
> > 
> > 1. When creating the first large file on a file system without
> >    EXT4_FEATURE_RO_COMPAT_LARGE_FILE feature.
> > 2. When re-sizing the file-system.
> > 3. When creating an xattr on a file-system without the
> >    EXT4_FEATURE_COMPAT_EXT_ATTR feature.
> > 4. When adding or deleting an orphan (because we update the 's_last_orphan'
> >    superblock field).
> > 
> > This function, however, falls back to just marking the superblock as dirty
> > if the file-system has no journal. This means that we delay the actual
> > superblock I/O submission by 5 seconds (roughly speaking). Namely, the
> > 'sync_supers()' kernel thread will call 'ext4_write_super()' later, where
> > we actually will submit the superblock down to the media.
> > 
> > However:
> > 1. For cases 1-3 it does not add any value to delay the I/O submission. These
> >    events are rare and we may just commit submit the superblock for
> >    asynchronous I/O right away.
> > 2. For case 4 - similarly, not terribly frequent event in most of workloads.
> >    It should be good enough to just submit asynchronous superblock write-out.
>   Well, it happens for every inode being truncated / deleted to it can be
> rather frequent. That's why I wanted to have now == 1 case everywhere -
> i.e. just recompute the checksum and do mark_buffer_dirty(). I'd just
> remove the 'now' test in this patch and then in patch 5 remove the now
> argument from the function and callers as you did.

It looked logical to me to use 'ext4_commit_super()' always and remove
'now' and marking the buffer dirty directly. Just because I thought the
speed difference should be nearly 0, and 'ext4_commit_super()' is doing
some error checking. But you seem to suggest to do the opposite, and I
do not understand why would that be better. So I dropped this change so
far.

I've sent v5 where I basically only changed the commit message in patch
3 and dropped patch 5. In patch 3 I've explicitly indicated that we'll
do more checksum calculations, but I think this is OK acceptable.

Thanks!

-- 
Best Regards,
Artem Bityutskiy

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