nodiratime [LWN.net]

nodiratime

Posted Aug 9, 2007 16:17 UTC (Thu) by corbet (editor, #1)
In reply to: nodiratime by elanthis
Parent article: Once upon atime

noatime implies nodiratime - it's a superset. You can use nodiratime by itself to turn off the updates on directories only. As you note, it does not seem to be widely used.

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Does noatime imply nodiratime?

Posted Aug 9, 2007 20:17 UTC (Thu) by tarvin (subscriber, #4412) [Link]

Are you sure about noatime implying nodiratime?

mount(8) states:
noatime Do not update inode access times on this file system[...] nodiratime Do not update directory inode access times on this filesystem.

while mount(2) puts it this way:
MS_NOATIME Do not update access times for (all types of) files on this file system. MS_NODIRATIME Do not update access times for directories on this file system.

I wonder how to interpret mount(2)'s (all types of) files for noatime: Is a directory considered a file in this context?

Does noatime imply nodiratime?

Posted Aug 9, 2007 20:39 UTC (Thu) by corbet (editor, #1) [Link]

Yep, I'm sure. When in doubt, use the source. From touch_atime() in fs/inode.c:

void touch_atime(struct vfsmount *mnt, struct dentry *dentry)
{
        /* ... */
        if (inode->i_flags & S_NOATIME)
                return;
        if (IS_NOATIME(inode))
                return;
        if ((inode->i_sb->s_flags & MS_NODIRATIME) && S_ISDIR(inode->i_mode))
                return;

So if NOATIME is set, the NODIRATIME flag is never even checked.

Does noatime imply nodiratime?

Posted Aug 10, 2007 6:41 UTC (Fri) by tarvin (subscriber, #4412) [Link]

OK. I've been using noatime systematically for years. And then I read about a prominent kernel developer like Molnar using both noatime and nodiratime, making me worried: Have I been missing out on I/O-performance for years?

- But your definite statement is comforting, thanks. I'll keep using just "noatime".

Does noatime imply nodiratime?

Posted Aug 10, 2007 14:26 UTC (Fri) by jzbiciak (✭ supporter ✭, #5246) [Link]

Actually, Andrew Morton actually corrected Ingo on this point:

From: Andrew Morton 
To:	Ingo Molnar 
Subject: Re: [PATCH 00/23] per device dirty throttling -v8
Date:	Sun, 5 Aug 2007 00:29:34 -0700

On Sun, 5 Aug 2007 09:21:41 +0200 Ingo Molnar [email blocked] wrote:

> even on a noatime,nodiratime filesystem

noatime is a superset of nodiratime, btw.

I trust Andrew on this point. :-)