kernel panic and boot once

ref: http://nfolamp.wordpress.com/2010/08/12/configuring-grub-to-boot-a-fallback-kernel/

this is a RHEL5 system.  Here is my grub.conf file.

default=0 timeout=10 title RHEL known *good* kernel root (hd0,0) kernel /vmlinuz-2.6.18-164.el5 ro root=/dev/OS/vol1  rhgb quiet initrd /initrd-2.6.18-164.el5.img savedefault   title RHEL *new* kernel root (hd0,0) kernel /vmlinuz-2.6.18-194.8.1.el5 ro root=/dev/OS/vol1 rhgb quiet panic=5 initrd /initrd-2.6.18-194.8.1.el5.img

My “untested” kernel is the second item listed.  The kernel param panic=5 will cause the system to automatically reset after 5 seconds following the event of a kernel panic.  Otherwise, the server would be stuck in a crashed state until someone could physically intervene and power cycle the server.

Now comes the interesting part.  We need to tell grub to set the “untested” kernel as the default and to boot it only once.  We can do this from the grub shell.

# grub   grub> savedefault --default=1 --once savedefault --default=1 --once grub> quit

As you can see I tell grub to save the “untested” kernel (–default=1) as the default for one-time only (–once).