zfs discuss - Feb 2007 - zfs legacy filesystem remounted rw: atime temporary off?

I have my /usr filesystem configured as a zfs filesystem,
using a legacy mountpoint.  I noticed that the system boots
with atime updates temporarily turned off (and doesn''t record
file accesses in the /usr filesystem):

# df -h /usr
Filesystem             size   used  avail capacity  Mounted on
files/usr-b57           98G   2.1G    18G    11%    /usr

# zfs get atime files/usr-b57
NAME           PROPERTY  VALUE          SOURCE
files/usr-b57  atime     off            temporary


That is, when a zfs legacy filesystem is mounted in
read-only mode, and then remounted read/write,
atime updates are off:

# zfs create -o mountpoint=legacy files/foobar

# mount -F zfs -o ro files/foobar /mnt

# zfs get atime files/foobar
NAME          PROPERTY  VALUE         SOURCE
files/foobar  atime     on            default

# mount -F zfs -o remount,rw files/foobar /mnt

# zfs get atime files/foobar
NAME          PROPERTY  VALUE         SOURCE
files/foobar  atime     off           temporary


Is this expected behaviour?

It works if I remount with the "atime" option:

# mount -F zfs -o remount,rw,atime files/foobar /mnt

# zfs get atime files/foobar
NAME          PROPERTY  VALUE         SOURCE
files/foobar  atime     on            default
 
 
This message posted from opensolaris.org

J?rgen Keil wrote:
> I have my /usr filesystem configured as a zfs filesystem,
> using a legacy mountpoint.  I noticed that the system boots
> with atime updates temporarily turned off (and doesn''t record
> file accesses in the /usr filesystem):
> 
> # df -h /usr
> Filesystem             size   used  avail capacity  Mounted on
> files/usr-b57           98G   2.1G    18G    11%    /usr
> 
> # zfs get atime files/usr-b57
> NAME           PROPERTY  VALUE          SOURCE
> files/usr-b57  atime     off            temporary
> 
> 
> That is, when a zfs legacy filesystem is mounted in
> read-only mode, and then remounted read/write,
> atime updates are off:
> 
> # zfs create -o mountpoint=legacy files/foobar
> 
> # mount -F zfs -o ro files/foobar /mnt
> 
> # zfs get atime files/foobar
> NAME          PROPERTY  VALUE         SOURCE
> files/foobar  atime     on            default
> 
> # mount -F zfs -o remount,rw files/foobar /mnt
> 
> # zfs get atime files/foobar
> NAME          PROPERTY  VALUE         SOURCE
> files/foobar  atime     off           temporary
> 
> 
> Is this expected behaviour?

I suspect it''s related to this bug:

http://bugs.opensolaris.org/view_bug.do?bug_id=6498096

which is "zfs noatime broken with legacy mount".

I started fixing this a while back but never finished
it.  Is this causing you pain, or is it just something
you noticed?

-Chris

> > That is, when a zfs legacy filesystem is mounted in
> > read-only mode, and then remounted read/write,
> > atime updates are off:
> > 
> > # zfs create -o mountpoint=legacy files/foobar
> > 
> > # mount -F zfs -o ro files/foobar /mnt
> > 
> > # zfs get atime files/foobar
> > NAME          PROPERTY  VALUE         SOURCE
> > files/foobar  atime     on            default
> > 
> > # mount -F zfs -o remount,rw files/foobar /mnt
> > 
> > # zfs get atime files/foobar
> > NAME          PROPERTY  VALUE         SOURCE
> > files/foobar  atime     off           temporary
> > 
> > 
> > Is this expected behaviour?
> 
> 
> I suspect it''s related to this bug:
> 
> http://bugs.opensolaris.org/view_bug.do?bug_id=6498096
> 
> which is "zfs noatime broken with legacy mount".
>
> I started fixing this a while back but never finished
> it.  Is this causing you pain, or is it just something
> you noticed?
I noticed that, and the atime remount behaviour doesn''t seem to be
intuitive.


I had a look at access time stamps to determine if a certain command
from /usr has recently run, when I found out that - for the above
reason - access time stamps didn''t work at all on my zfs /usr
filesystem.  Note that /usr is mounted early in read only mode by
/lib/svc/method/fs-root, and later remounted read/write
by /lib/svc/method/fs-usr



I guess I could hack around the issue by adding an "atime" 
option to /etc/vfstab for the /usr mount, but I havn''t tried
this yet.