Date: Thu, 04 Dec 2008 15:20:20 -0800 From: "Kevin Oberman" <oberman@es.net> To: josh.carroll@gmail.com Cc: Alexandr Pakhomov <pahom.rt@gmail.com>, Mike Tancsa <freebsd-stable@freebsd.org>, Steve Polyack <korvus@comcast.net> Subject: Re: repeatable crash on RELENG7 Message-ID: <20081204232020.7289A45019@ptavv.es.net> In-Reply-To: Your message of "Thu, 04 Dec 2008 17:11:46 EST." <8cb6106e0812041411t13ca9ffcjc4cc66a33ef43133@mail.gmail.com>
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--==_Exmh_1228432820_12904P Content-Type: text/plain; charset=us-ascii Content-Disposition: inline > Date: Thu, 4 Dec 2008 17:11:46 -0500 > From: "Josh Carroll" <josh.carroll@gmail.com> > > > It seems that the term "swap-backed" is misleading for some people. It does > > NOT mean your md(4) device will be constantly swapping to disk (and the man > > page does an alright job of relaying this). It simply means that generally > > available memory will be used, and so will swap iff available memory happens > > to drop low enough. > > > > The bottom line in my experience with md(4) devices greater than ~100MB is > > that "swap-backed" is always reliable, while malloc'd md(4) devices will > > cause unpredictable kernel panics. > > Using -t swap instead of -t malloc will prevent a panic, but creating > an md greater than the size of the VM available and filling it will > cause resource exhaustion and OOM will kick in and start killing > processes, right? So while it won't panic the box, I still wouldn't > consider it safe to use unless the size of the md is chosen carefully. > Someone please correct me if I'm wrong, but I think that'd be the > behavior if an md with -t swap is used, right? Yes, but the VM available is just a bit larger than the amount of KVM. you still need at least a bit of sanity when you create (and fill) an MD device. For performance reasons, I suggest keeping the size of an MD so that it will fit in RAM. Paging in and out is rather painfully slow. -- 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 --==_Exmh_1228432820_12904P Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (FreeBSD) Comment: Exmh version 2.5 06/03/2002 iD8DBQFJOGW0kn3rs5h7N1ERAh9jAJoDr0hX/7rTzLodd6wF8gm0MsfdLACfebHl FSfNU/zkkOoIxPN91Owe/V0= =hyxw -----END PGP SIGNATURE----- --==_Exmh_1228432820_12904P--
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