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XFS and atime update strangeness

To: xfs@xxxxxxxxxxx
Subject: XFS and atime update strangeness
From: Michael Weissenbacher <mw@xxxxxxxxxxxx>
Date: Fri, 19 Jun 2009 11:36:20 +0200
User-agent: Thunderbird 2.0.0.21 (X11/20090409)
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.

I haven't found anything in the official documentation stating this. The only 
reference i found was a while back in the mailing list:
http://oss.sgi.com/archives/xfs/2007-10/msg00171.html

I think there should be at least some information about this behavior in the 
official docs and the FAQ. IMO This feature/bug of XFS also collides with the 
recent change to make relatime the default in kernel 2.6.30. Is there any way 
to enable permanent atime updates in case someone wants them?

Michael

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