Label question...why does ufs label vanish on mount?

[Kevin Oberman]

> Date: Wed, 13 Oct 2010 13:26:30 +0200
> From: Pawel Jakub Dawidek <pjd at FreeBSD.org>
> Sender: owner-freebsd-stable at freebsd.org
> 
> On Wed, Oct 13, 2010 at 11:47:41AM +0200, Stefan Bethke wrote:
> > Am 13.10.2010 um 10:20 schrieb Pawel Jakub Dawidek:
> > 
> > > On Tue, Oct 12, 2010 at 11:33:11PM -0700, Jeremy Chadwick wrote:
> > >> On Wed, Oct 13, 2010 at 08:29:06AM +0200, Stefan Bethke wrote:
> > >>> That explains the mechanism, but not the rationale.  Or is it just an unintended consequence?  And how is da2p1 different from ufs/mylabel?  (Mount da2p1 and ufs/mylabel is removed, but not the other way around.)
> > >> 
> > >> Pulling in pjd@ who can probably shed some light on this.
> > > 
> > > The ufs/mylabel provider is based on da2p1, that's why opening da2p1
> > > makes ufs/mylabel to be removed and not the other way around.
> > > 
> > > The ufs/mylabel provider was created, because when da2p1 provider was
> > > created and LABEL class tasted it, it discovered that this provider
> > > contains UFS file system with 'mylabel' volume label, so the LABEL class
> > > created ufs/mylabel provider. Now when you open da2p1 for writing, the
> > > LABEL class destroys ufs/mylabel, because you may decide to change
> > > metadata on da2p1, for example you may choose to destroy UFS in there or
> > > change the volume label. When write open count on da2p1 goes down to
> > > zero, the LABEL class will be given da2p1 provider for tasting once
> > > again, so it can rediscover (possibly modified) volume label.
> > > 
> > > The class may choose to ignore the spoil event from GEOM (it is send on
> > > first open for write), but if it isn't based on autodiscovering
> > > metadata. For example the NOP class ignores this event, because it
> > > doesn't care about metadata of provider it is based on.
> > > 
> > > If we choose to ignore the spoil event in the LABEL class we will end up
> > > with stale info, eg. open da2p1 for writing, change its volume label and
> > > mount it and you will still have old label in /dev/ufs/.
> > 
> > Thanks a lot (and also to Andrey), that really makes it clear to me!
> > 
> > I just wish there was an easy way to keep the labels around even while someone has the provider open for writing, but I now understand that this requires some significant changes.
> 
> The changes aren't significant. We could eventually ignore spoil event
> and keep labels around even when underlying provider is opened for
> writing risking the label is stale. We could then only update or remove
> the label on retaste event (when underlying provider's open write count
> goes down to zero).
> 
> Currently when we do, eg.
> 
> 	# dd if=/dev/zero of=/dev/da2p1 bs=1m
> 
> This is happening:
> 
> 	# dd(1) opens da2p1 for writing
> 	# GEOM sends spoil event to all consumers of da2p1
> 	# LABEL class destroys /dev/ufs/mylabel provider
> 	# dd(1) finishes and closes da2p1
> 	# GEOM sends taste event to all GEOM classes
> 	# LABEL class finds no metadata and ignores da2p1
> 
> With the new world order this would look like this:
> 
> 	# dd(1) opens da2p1 for writing
> 	# GEOM sends spoil event to all consumers of da2p1
> 	# LABEL class ignores spoil event
> 	# dd(1) finishes and closes da2p1
> 	# GEOM sends taste event to all GEOM classes
> 	# LABEL class finds no metadata on da2p1 and destroys /dev/ufs/mylabel

Thanks! Thi explains most of what I see, but there is one thing that is
not explained. That is the devd showing a CREATE for the partition
(e.g. da0s2d) every time the partition is either mounted or umounted,
regardless of whether it is mounted by device or label. There is never a
DESTROY event. this is both counter-intuitive, but hard to program
around.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751