From owner-freebsd-stable@FreeBSD.ORG Wed Oct 13 20:35:40 2010 Return-Path: Delivered-To: stable@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 0055A1065823; Wed, 13 Oct 2010 20:35:39 +0000 (UTC) (envelope-from oberman@es.net) Received: from mailgw.es.net (mail1.es.net [IPv6:2001:400:201:1::2]) by mx1.freebsd.org (Postfix) with ESMTP id CC1A88FC15; Wed, 13 Oct 2010 20:35:39 +0000 (UTC) Received: from ptavv.es.net (ptavv.es.net [IPv6:2001:400:910::29]) by mailgw.es.net (8.14.3/8.14.3) with ESMTP id o9DKZd0l028833 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NOT); Wed, 13 Oct 2010 13:35:39 -0700 Received: from ptavv.es.net (localhost [127.0.0.1]) by ptavv.es.net (Tachyon Server) with ESMTP id 240861CC3E; Wed, 13 Oct 2010 13:35:39 -0700 (PDT) To: Pawel Jakub Dawidek In-reply-to: Your message of "Wed, 13 Oct 2010 13:26:30 +0200." <20101013112630.GA1727@garage.freebsd.pl> Date: Wed, 13 Oct 2010 13:35:39 -0700 From: "Kevin Oberman" Message-Id: <20101013203539.240861CC3E@ptavv.es.net> Cc: stable@freebsd.org, "Andrey V. Elsukov" , Stefan Bethke , Jeremy Chadwick Subject: Re: Label question...why does ufs label vanish on mount? X-BeenThere: freebsd-stable@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Production branch of FreeBSD source code List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 13 Oct 2010 20:35:40 -0000 > Date: Wed, 13 Oct 2010 13:26:30 +0200 > From: Pawel Jakub Dawidek > Sender: owner-freebsd-stable@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@es.net Phone: +1 510 486-8634 Key fingerprint:059B 2DDF 031C 9BA3 14A4 EADA 927D EBB3 987B 3751