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Date:      Tue, 14 Oct 2025 18:53:41 +0000
From:      bugzilla-noreply@freebsd.org
To:        bugs@FreeBSD.org
Subject:   [Bug 290235] bsdinstall: swap/dump partition too small to store kernel dump
Message-ID:  <bug-290235-227@https.bugs.freebsd.org/bugzilla/>

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D290235

            Bug ID: 290235
           Summary: bsdinstall: swap/dump partition too small to store
                    kernel dump
           Product: Base System
           Version: 16.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: emaste@freebsd.org

bsdinstall sets dumpdev=3DAUTO if kernel core dumps are enabled, which uses=
 the
first available swap device as a kernel dump device.

The default swap size has long been set via the algorithm in
usr.sbin/bsdinstall/partedit/part_wizard.c (and later applied to
usr.sbin/bsdinstall/scripts/zfsboot also):

static intmax_t
swap_size(intmax_t available)
{
        intmax_t swapsize;
        unsigned long maxswap;
        size_t sz;

        swapsize =3D MIN(available/20, 4*1024*1024*1024LL);
        sz =3D sizeof(maxswap);
        if (sysctlbyname("vm.swap_maxpages", &maxswap, &sz, NULL, 0) =3D=3D=
 0)
                swapsize =3D MIN(swapsize, (intmax_t)maxswap * getpagesize(=
));

        return (swapsize);
}

I.e., it is set to 1/20th of the amount of available memory, with a minimum
bound of 4GB and a maximum bound of the maximum amount of swap. The 4GB low=
er
bound will apply until the machine has at least 80GB of memory.

On my test laptop with 32GB of memory I often experience dump failure due t=
o an
insufficient dump partition size.

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