pfSense

ZFS guided auto-install, selected zfs-mirror, two disks.
As a result we have two swap partitions in fstab

1. Device Mountpoint FStype Options Dump Pass#
   /dev/gpt/efiboot0 /boot/efi msdosfs rw 2 2
   /dev/ada0p3 none swap sw 0 0
   /dev/ada1p3 none swap sw 0 0

@

Original rc.dumpon consists function

@
dumpon_try() {
if /sbin/dumpon "${1}" ; then # Create a symlink in devfs for savecore
echo "Using ${1} for dump device."
ln -fs "${1}" /dev/dumpdev
return 0
fi
echo "Unable to specify $1 as a dump device."
return 1
}

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This function overwrites the symlink for dump device with last one tried device.
As a result we have symlink, but no dump for the savecore, because the dump is saved on the first partition, not the last one.
You can test it by entering command in console.

sysctl debug.kdb.enter=1


As a workaround, I've modified code

@
dumpon_try() {
local dump_device="$1"

if /sbin/dumpon "$dump_device" ; then
    # Create a symlink in devfs for savecore only if it doesn't exist
        if [ ! -e "/dev/dumpdev" ]; then
            ln -s "$dump_device" /dev/dumpdev
        fi
    # Print a message for each dump device found
        echo "Using $dump_device for dump device." 
        return 0
    fi

echo "Unable to specify $dump_device as a dump device." 
    return 1
}

@