linux-ext4 - Re: [PATCH] ext4: turn on i_version updates by default

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Date:	Tue, 15 May 2012 11:50:37 -0700
From:	djwong <djwong@...ibm.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	"Theodore Ts'o" <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] ext4: turn on i_version updates by default

On Tue, May 15, 2012 at 01:33:40PM -0400, J. Bruce Fields wrote:
> On Mon, May 14, 2012 at 11:27:42AM -0600, Andreas Dilger wrote:
> > On 2012-05-14, at 9:23 AM, J. Bruce Fields wrote:
> > > On Mon, May 14, 2012 at 09:02:12AM -0600, Andreas Dilger wrote:
> > >> On 2012-05-14, at 8:06, "J. Bruce Fields" <bfields@...ldses.org> wrote:
> > >>> knfsd needs i_version updates on, as will userspace nfs servers and
> > >>> probably others.
> > >>> 
> > >>> The only effects are that inode->i_version is bumped (under the i_lock)
> > >>> in more places, and that ->dirty_inode(I_DIRTY_DATASYNC) may be called
> > >>> more frequently than once per jiffy on write (see file_update_time).
> > >>> However the latter appears to be mostly a no-op in that case.
> > >> 
> > >> I thought this can have noticeable performance impact, since ext4_mark_inode_dirty() is quite heavyweight?
> > > 
> > > There's no reason it should be, should it, if we already just dirtied
> > > the inode a moment ago?
> > 
> > Ideally not, but the way ext[34]_mark_inode_dirty() is implemented
> > is that it copies the whole in-core inode to the on-disk inode every
> > time it is marked dirty.  That ensures that the on-disk inode is
> > up-to-date when the journal flushes the blocks to disk, but is not
> > an ideal implementation.  It has been this way since the first ext3
> > implementation was done.
> > 
> > As a result, dirtying the inode very frequently for ext[34] is
> > currently expensive and should be avoided.
> > 
> > I _think_ that the ext4 metadata checksum patches have changed this
> > to only flag the inode dirty and run a pre-commit callback to copy
> > the in-core inode to the on-disk inode.  I'm not sure what the
> > current status of that patch is, nor how easily it could be split
> > from that patch series and land separately.
> 
> I did some searching, found a couple of versions of the metadata
> checksum patches, but no patch that looked like what you're describing.
> Any idea where that is?

That _was_ going to be the basis of phase 2 of my metadata checksum patchset,
but since I've been moved to other projects, I don't see that on my plate in
the near future. :/

(tldr: It doesn't exist.)

--D
> 
> --b.
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