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

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Message-Id: <9124E59E-2479-4C32-A528-3237B48DEC01@dilger.ca>
Date:	Mon, 14 May 2012 09:02:12 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	"J. Bruce Fields" <bfields@...ldses.org>
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 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?

> So, simplify our life and just keep this feature turned on all the time.
> 
> Signed-off-by: J. Bruce Fields <bfields@...hat.com>
> ---
> fs/ext4/super.c |    3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> If people are worried that the performance impact isn't obvious, I'll
> find the time somehow to test this properly....
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index e1fb1d5..a99f827 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1483,7 +1483,7 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
>        sbi->s_mount_flags |= EXT4_MF_FS_ABORTED;
>        return 1;
>    case Opt_i_version:
> -        sb->s_flags |= MS_I_VERSION;
> +        /* no-op; this is on by default now */
>        return 1;
>    case Opt_journal_dev:
>        if (is_remount) {
> @@ -2979,6 +2979,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>        goto out_free_orig;
>    }
>    sb->s_fs_info = sbi;
> +    sb->s_flags |= MS_I_VERSION;
>    sbi->s_mount_opt = 0;
>    sbi->s_resuid = EXT4_DEF_RESUID;
>    sbi->s_resgid = EXT4_DEF_RESGID;
> -- 
> 1.7.9.5
> 
> --
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