Rescuing Linux Systems--Generic and Distribution-Specific Safety Nets - page 4
Sending Out an SOS
Most Linux distributions enable you to create a rescue floppy as part of the installation process. These rescue floppies are primarily designed to help you recover from simple boot configuration or boot loader problems, such as forgetting to update your boot loader configuration files after building a new kernel, forgetting to run LILO after such an update (if you're using LILO as your boot loader), and so on.
The primary drawback of the rescue disks created by most Linux distributions is that they don't provide tools to help you recover from more serious problems. For example, the boot configuration file on rescue disks created with the "mkbootdisk" script that is provided with Red Hat Linux contains an entry for the location of the root filesystem on the system where it was created. You can therefore only use this type of rescue disk "out of the box" to boot systems that have the same partitioning scheme and use the same general type of hardware as the system on which it was created.
When using floppy rescue disks such as Red Hat's, you can usually specify the "root=/dev/whatever" option at the rescue disk's LILO prompt if the partitioning scheme is different on the system that you are trying to rescue, replacing "whatever" with the name correct root partition for the system that you are trying to rescue. However, since this type of rescue disk contains a kernel image taken from the system on which it was created, it may not support the hardware in your other systems. For example, a rescue disk created on a system without SCSI support won't help you rescue a SCSI-based system unless the kernel on the rescue disk system has SCSI support compiled in, since most rescue floppies do not include loadable kernel modules. This isn't surprising--after all, they have to fit on a floppy.
Floppy-based rescue disks that are simply designed to help you boot your system usually do not include any utilities to enable you to repair a more seriously damaged system. To continue with the Red Hat example, rescue disks created with the "mkbootdisk" script only contain a boot block, kernel, and associated configuration files. They depend on being able to locate and mount your system's root filesystem in order to find the tools that you may need to completely "rescue" that system. If your system's root file system is corrupted or otherwise damaged, you may not be able to access those tools.
Most of today's Linux distributions include a "rescue" mode on their boot CD that enables you to boot a generic kernel and provides access to critical tools such as fsck and the utilities used to create and write boot configuration information. The next section explores the rescue capabilities of the boot CDs for a variety of Linux distributions.
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