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

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Date:	Mon, 14 May 2012 23:08:19 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	"J. Bruce Fields" <bfields@...ldses.org>,
	"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, 2012-05-14 at 09:02 -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?
> 
> This is one of the reasons that the i_version update is conditional. If someone is exporting a filesystem from userspace the should be able to turn this on as a mount option, and knfsd could do it from inside the kernel. Why add overhead when it is not needed?

No. Having knfsd doing something like that under the covers is a BAD
idea. It is way too easy to get into situations where someone starts
changing files after the mount and before knfsd is started. As soon as
you allow files to be changed without i_version changing, then you are
setting yourself up for future corruption.

Ideally, an NFSv4-exported filesystem should be required to set the
tune2fs mount_opts to include the 'i_version' flag to make it hard to
inadvertently mount that filesystem without it.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com