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Re: XFS and atime update strangeness

To: Michael Weissenbacher <mw@xxxxxxxxxxxx>
Subject: Re: XFS and atime update strangeness
From: Felix Blyakher <felixb@xxxxxxx>
Date: Tue, 23 Jun 2009 10:01:22 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <4A3B5C14.9070101@xxxxxxxxxxxx>
References: <4A3B5C14.9070101@xxxxxxxxxxxx>
On Jun 19, 2009, at 4:36 AM, Michael Weissenbacher wrote:

Hi XFS-List!
As i have been playing around with atime settings recently, i noticed that XFS handles atime updates very differently than other filesystems (at least different than ext2/3/4 and reiserfs). In fact it does not record atime changes permanently, it seems that atimes are only stored in the inode cache. I was testing this on a Gentoo Box with kernel 2.6.28-gentoo-r5 and 2.6.30 Vanilla.


How to reproduce?

invincible usr # ls -lu portage/header.txt
-rw-r--r-- 1 root root 121 2009-06-18 22:06 portage/header.txt
invincible usr # date
Fri Jun 19 11:24:53 CEST 2009
invincible usr # cat portage/header.txt > /dev/null
invincible usr # ls -lu portage/header.txt
-rw-r--r-- 1 root root 121 2009-06-19 11:24 portage/header.txt
invincible usr # umount portage && mount portage
invincible usr # ls -lu portage/header.txt
-rw-r--r-- 1 root root 121 2009-06-18 22:06 portage/header.txt

So after umount && mount the atime instantly went back to the old value. I can also sometimes reproduce it by doing "sync && echo 3 > / proc/sys/vm/drop_caches". If i wait for a day the atime also reverts back. Rebooting the machine has the same effect.

Right. All this says that atime is updated in the incore inode,
but never makes it to disk.
Agree, it's easily reproducible in all latest releases.

I haven't found anything in the official documentation stating this.

That's definitely not a feature to be documented. It has to
be fixed.

The only reference i found was a while back in the mailing list:
http://oss.sgi.com/archives/xfs/2007-10/msg00171.html

And again a year later

http://oss.sgi.com/archives/xfs/2008-10/msg02064.html

I think there should be at least some information about this behavior in the official docs and the FAQ. IMO This feature/bug

bug :)

of XFS also collides with the recent change to make relatime the default in kernel 2.6.30.

If we properly sync the xfs inode to disk, whenever linux inode
is dirtied with updated atime, it will work exactly as in VFS
(whatever you see before remount), i.e. as relatime.

For this we need the patch suggested by Dave Chinner:

http://oss.sgi.com/archives/xfs/2008-10/msg02102.html

Is there any way to enable permanent atime updates in case someone wants them?

That's another good request I'd like to address as well. The
above mentioned patch just makes it behave as relatime.

Thanks for bringing this up.

Felix



Michael

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