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

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Message-ID: <20120515173340.GA10064@fieldses.org>
Date:	Tue, 15 May 2012 13:33:40 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	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 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?

--b.
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