grub.conf内核启动失败的fallback设置范例如下，内核启动时候，我们默认使用default 1 CentOS (2.6.18-308.4.1.el5xen)启动，如果启动失败，系统会尝试 fallback 0 CentOS (2.6.18-308.4.1.el5)， 这对于远程管理服务器来说，特别是新安装内核，修改 grub.conf启动文件重启机器的时候，特别保险。ctohome.com推荐大家都设置这个参数

[[email protected]]# cat /etc/grub.conf
# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/sda2
#          initrd /initrd-version.img
#boot=/dev/sda
default 1
timeout 5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
fallback 0
title CentOS (2.6.18-308.4.1.el5)
        root (hd0,0)
        kernel /vmlinuz-2.6.18-308.4.1.el5 ro root=LABEL=/ rhgb quiet
        initrd /initrd-2.6.18-308.4.1.el5.img
title CentOS (2.6.18-308.4.1.el5xen)
        root (hd0,0)
        kernel /xen.gz-4.1.2
        module /vmlinuz-2.6.18-308.4.1.el5xen ro root=LABEL=/ rhgb quiet
        module /initrd-2.6.18-308.4.1.el5xen.img
title CentOS (2.6.18-308.el5)
        root (hd0,0)
        kernel /vmlinuz-2.6.18-308.el5 ro root=LABEL=/ rhgb quiet
        initrd /initrd-2.6.18-308.el5.img
[[email protected]]# uname -a
Linux host621.com 2.6.18-308.4.1.el5 #1 SMP Tue Apr 17 17:08:00 EDT 2012 x86_64 x86_64 x86_64 GNU/Linux
[[email protected]]#