Date: Mon, 1 Sep 2008 15:27:38 -0700 From: Jeremy Chadwick <koitsu@FreeBSD.org> To: Kevin Oberman <oberman@es.net> Cc: Derek =?iso-8859-1?B?S3VsacU/c2tp?= <takeda@takeda.tk>, Michael <freebsdports@bindone.de>, freebsd-stable@freebsd.org Subject: Re: bin/121684: : dump(8) frequently hangs Message-ID: <20080901222738.GA17793@icarus.home.lan> In-Reply-To: <20080901213856.GA17155@icarus.home.lan> References: <200809011336.m81Da5BT046532@lava.sentex.ca> <20080901160013.0005F4500F@ptavv.es.net> <20080901213856.GA17155@icarus.home.lan>
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On Mon, Sep 01, 2008 at 02:38:56PM -0700, Jeremy Chadwick wrote: > ... I'll try to find the thread (it was a year or so ago) > where a developer told me more or less what was going on. The problem > was that UFS2 snapshot generation, over time, becomes slower and slower > to generate (this is what dump does on UFS2 systems, with or without the > -L flag), and is a known design issue. The issue I'm describing above is fully discussed here. Be sure to read the threads I reference in my mail, as well as Eric Anderson and Peter Jeremy's replies: http://lists.freebsd.org/pipermail/freebsd-hackers/2007-October/021985.html There's also a statement from Kris Kennaway in one of the above referenced threads, who states that "UFS2 snapshots (e.g. dump -L) were not intended to be used this way": http://lists.freebsd.org/pipermail/freebsd-stable/2007-January/032070.html In that same thread, Doug Ambrisko provided a patch which supposedly made UFS2 snapshot generation significantly better for him, specifically with regards to dump "getting wedged": http://lists.freebsd.org/pipermail/freebsd-stable/2007-January/032250.html Another thread discussing this problem ("lock up when dump is used"): http://lists.freebsd.org/pipermail/freebsd-stable/2008-June/042823.html And finally, an issue I encountered (rebooting a system in the middle of dump -L) was documented here: http://lists.freebsd.org/pipermail/freebsd-stable/2007-September/036916.html I'll be adding these issues to my FreeBSD wiki page momentarily. I think folks should reconsider use of dump when UFS2 filesystems are chosen. This goes against what the O'Reilly "Backup & Recovery" book concludes, and what Elizabeth Zwicky concluded in her backup/recovery torture tests (albeit 17 years ago, but the document is referenced in the FreeBSD handbook itself): http://www.coredumps.de/doc/dump/zwicky/testdump.doc.html If you're using older UFS1 filesystems, dump should be fine, as there's no snapshot generation supported there. If the problem is truly with UFS2 snapshot generation, then I'd like to know why dump bothers with spitting out a warning when -L is used, more or less inducing people to modify their scripts to use the -L flag. The commit which induced this behaviour is from November 2002, and modified in April 2004 to advocate -L usage even more: http://www.freebsd.org/cgi/cvsweb.cgi/src/sbin/dump/main.c.diff?r1=1.42;r2=1.43;f=h http://www.freebsd.org/cgi/cvsweb.cgi/src/sbin/dump/main.c.diff?r1=1.57;r2=1.58;f=h -- | Jeremy Chadwick jdc at parodius.com | | Parodius Networking http://www.parodius.com/ | | UNIX Systems Administrator Mountain View, CA, USA | | Making life hard for others since 1977. PGP: 4BD6C0CB |
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