repeatable crash on RELENG7

[Kevin Oberman]

> Date: Thu, 4 Dec 2008 17:11:46 -0500
> From: "Josh Carroll" <josh.carroll at 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 at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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