[Bug 229764] Default settings allow system to wire all ram

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=229764

            Bug ID: 229764
           Summary: Default settings allow system to wire all ram
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: FreeBSD at ShaneWare.Biz

vfs.zfs.arc_max has a default of "all RAM less 1 GB, or one half of RAM,
whichever is more." - from handbook 19.6.1. Memory used by ARC is wired to
prevent it being swapped out.

vm.max_wired has a default setting of 30% of physical ram which limits what the
kernel allocates as wired.

Currently, these two values that limit the amount of wired ram do not interact
in any way. This means the default settings allow more than 100% of physical
ram to be allocated as wired. This is an unrecoverable situation.

The default for arc_max should be reduced to less than 70% of physical ram.

I believe that setting arc_max should also be prevented from going higher than
physical ram minus max_wired minus a safety buffer.

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