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Linux Tutorial - Solving Problems - Solving Problems Yourself - Crash Recovery
  Problem Solving ---- Hardware Diagnostic Tools  

Crash Recovery

If your system crashes, the most important thing is to prevent further damage. Hopefully, the messages that you see on your screen will give you some indication of what the problem is so that you can correct it. (For example, timeout errors on your hard disk might mean it's time to buy a new hard disk.)

Emergency Floppies

It may happen that the system crash you just experienced no longer allows you to boot your system. What then? The easiest solution (at least the easiest in terms of figuring out what to do) is reinstalling. If you have a recent backup and your tape drive is fairly fast, this is a valid alternative, provided there is no hardware problem causing the crash.

In an article I once wrote, I compared a system crash to an earthquake. The people who did well after the 1989 earthquake in Santa Cruz, Ca were those who were most prepared. The people who do well after a system crash are also those who are best prepared. As with an earthquake, the first few minutes after a system crash are crucial. The steps you take can make the difference between a quick, easy recovery and a forced re-install.

In previous sections, I talked about the different kinds of problems that can happen on your system, so there is no need to go over them again here. Instead, I will concentrate on the steps to take after you reboot your system and find that something is wrong. It's possible that when you reboot all is well and it will be another six months before that exact same set of circumstances occurs. On the other hand, your screen may be full of messages as it tries to bring itself up again.

Because of the urgent nature of system crashes and the potential loss of income, I decided that this was one troubleshooting topic through which I would hold your hand. There is a set of common problems that occur after a system crashes that need to be addressed. Although the cause of the crash can be a wide range of different events, the result of the crash is small by comparison. With this and the importance of getting your system running again in mind, I am going to forget what I said about giving you cookbook answers to specific questions for this one instance.

Lets first talk about those cases in which you can no longer boot at all. Think back to our discussion of starting and stopping the system and consider the steps the system goes through when it boot. I talked about them in detail before, so I will only review them here as necessary to describe the problems.

As I mentioned, when you turn on a computer, the first thing is the Power-On Self-Test, or POST. If something is amiss during the POST, you will usually hear a series of beeps. Hopefully, there will be some indication on your monitor of what the problem is. It can be anything from incorrect entries in your CMOS to bad RAM. If not, maybe the hardware documentation mentions something about what the beeps mean.

When finished with the POST, the computer executes code that looks for a device from which it can boot. On a Linux system, this boot device will more than likely be the hard disk. However, it could also be a floppy or even a CD-ROM. The built-in code finds the active partition on the hard disk and begins to execute the code at the beginning of the disk. What happens if the computer cannot find a drive from which to boot depends on your hardware. Often a message will indicate that there is no bootable floppy in drive A. It is also possible that the system will simply hang.

If your hard disk is installed and it should contain valid data, your master boot block is potentially corrupt. If you created the boot/root floppy set as I told you to do, you can use fdisk from it to recreate the partition table using the values from your notebook. Load the system from your boot/root floppy set and run fdisk.

This is done from the hard disk. With the floppy in the drive, you boot your system. When you get to the Boot: prompt, simply press Enter. After loading the kernel, it prompts you to insert the root file system floppy. Do that and press Enter. A short time later, you are brought to a # prompt from which you can begin to issue commands.

When you run fdisk, you will probably see an empty table. Because you made a copy of your partition table in your notebook as I told you to do, simply fill in the values exactly as they were before. Be sure that you make the partition active as it was previously. Otherwise, you won't be able to boot, or you could still boot but you will corrupt your file system. When you exit fdisk, it will write out a copy of the master boot block to the beginning of the disk. When you reboot, things will be back to normal.

(I've talked to at least one customer who literally laughed at me when I told him to do this. He insisted that it wouldn't work and that I didn't know what I was talking about. Fortunately for me, each time I suggested it, it did work. However, I have worked on many machines where it didn't work. With a success rate of more than 90 percent, it's obviously worth a try.)

Table - Files Used in Problem Solving

Command Description
/bin/pstatReports system information
/bin/whoLists who is on the system
/bin/whodoDetermines what process each user is running
/etc/badblockChecks for bad spots on your hard disk
/bin/rpmDisplays information about install packages (also used to install and remove software)
/etc/dfCalculates available disk space on all mounted file
/etc/fdisk Creates and administers disk partitions
/etc/fsck Checks and cleans file systems
/etc/fuser Indicates which users are using particular files and file systems
/etc/ifconfig Configures network interface parameters
/etc/ps Reports information on all processes
/usr/adm/hwconfig probe for hardware
/var/log/wtmpLogin records
/usr/bin/cpioCreates archives of files
/usr/bin/lastIndicates last logins of users and teletypes
/usr/bin/lpstatPrints information about status of print service
/usr/bin/netstatAdministers network interfaces
/usr/bin/sarReports on system activity
/usr/bin/tarCreates archives of files
/usr/bin/wReports who is on the system and what they are doing

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