Some of you may not really care about swap. After all, Linux systems are typically very memory efficient, and swap is often barely touched. While often true on desktop systems, servers are another story. Because servers may handle unexpected stresses, such as runaway processes, denial of service attacks, or even the Slashdot effect, they need to have adequate high-speed swap so that they do not grind to a halt and possibly crash when all physical memory (and then some) is exhausted.

Still not convinced that this is a big deal? I’ll show you how easy it is to bring down a server by launching a massive amount of new processes. (Please, if you try this, do it only on a non-production server that you actually administer!) Let’s say you have two customized grep commands in /usr/bin, called bobgrep and jimgrep. Now, let’s assume that bobgrep is simply a shell script that calls the ELF executable jimgrep, as follows:

#!/bin/bash
jimgrep -r $*

Everything looks good so far, but what happens if jimgrep gets accidentally replaced with a symbolic link to bobgrep? Well, in that case, calling bobgrep or jimgrep will cause an infinite loop, causing hundreds of bash processes to be spawned in mere seconds. This actually happened to me once, and believe me, it hurt!

If a server doesn’t have adequate swap, a situation like this can cause the machine to lock up in much less than a minute. How do we fix the problem? One way is to increase the swap size beyond 128 MB. Fortunately for us, there is absolutely no 128 MB limit on swap under Linux 2.2.x+ and later kernels, as there was in the past. The current limits are approximately 2 GB on x86, PowerPC, and MC680x0 systems, 512 MB on MIPS systems, 128 GB on Alpha, and a whopping 3 Terabytes on UltraSparc platforms!

Swap partition size limits under modern Linux kernels

x86		    2 Gigabytes
PowerPC		    2 Gigabytes
Motorola 680x0	    2 Gigabytes
Sparc		    1 Gigabyte
MIPS		    512 Megabytes
Alpha		    128 Gigabytes
UltraSparc	    3 Terabytes

While it’s nice to be able to increase swap partition size to beyond 128 MB, how about increasing performance? Ideally, it would be nice if we could set up swap partitions in a RAID 0 stripe, so that reads and writes are equally distributed between all partitions. If these partitions are on separate drives and/or controllers, this will multiply swap file performance, allowing your servers to handle temporary memory usage “spikes” without getting dramatically bogged down.

Amazingly, all modern Linux kernels, by default (with no special kernel options or patches) allow you to parallelize swap, just like a RAID 0 stripe. By using the pri option in /etc/fstab to set multiple swap partitions to the same priority, we tell Linux to use them in parallel:

Set multiple swap partitions to the same priority

/dev/sda2	none	swap	sw,pri=3	0	0
/dev/sdb2	none	swap	sw,pri=3	0	0
/dev/sdc2	none	swap	sw,pri=3	0	0
/dev/sdd2	none	swap	sw,pri=1	0	0

In the above example, Linux will use swap partitions sda2, sdb2, and sdc2 in parallel. Since these partitions are on different drives, and possibly even different SCSI controllers, read and write throughput will nearly triple. The fourth swap partition, sdd2, will be used only after the first three partitions have been exhausted.

The pri option is really easy to use. The priority must be a number between 0 and 32767, with 32767 being the highest priority. The swap partitions will be used from highest priority to lowest priority, meaning that a partition with a priority of x will only be used only if all partitions with a priority greater than x are already full. If several partitions have the same priority, Linux will automatically parallelize access between them. This allows you to not only parallelize swap, but also prioritize access so that the partitions on the fastest drives (or regions of the drives) are used first. So, you can set up an emergency swap partition on an old, slower drive that will be used only if all high-speed swap is exhausted first.

#1: Use a Windows startup disk

One of the first things you should reach for when troubleshooting a Windows XP boot problem is a Windows startup disk. This floppy disk can come in handy if the problem is being caused when either the startup record for the active partition or the files that the operating system uses to start Windows have become corrupted.

To create a Windows startup disk, insert a floppy disk into the drive of a similarly configured, working Windows XP system, launch My Computer, right-click the floppy disk icon, and select the Format command from the context menu. When you see the Format dialog box, leave all the default settings as they are and click the Start button. Once the format operation is complete, close the Format dialog box to return to My Computer, double-click the drive C icon to access the root directory, and copy the following three files to the floppy disk:

• Boot.ini
• NTLDR
• Ntdetect.com

After you create the Windows startup disk, insert it into the floppy drive on the afflicted system and press [Ctrl][Alt][Delete] to reboot the computer. When you boot from the Windows startup disk, the computer will bypass the active partition and boot files on the hard disk and attempt to start Windows XP normally.

#2: Use Last Known Good Configuration

You can also try to boot the operating system with the Last Known Good Configuration feature. This feature will allow you to undo any changes that caused problems in the CurrentControlSet registry key, which defines hardware and driver settings. The Last Known Good Configuration feature replaces the contents of the CurrentControlSet registry key with a backup copy that was last used to successfully start up the operating system.

To use the Last Known Good Configuration feature, first restart the computer by pressing [Ctrl][Alt][Delete]. When you see the message Please select the operating system to start or hear the single beep, press [F8] to display the Windows Advanced Options menu. Select the Last Known Good Configuration item from the menu and press [Enter].

Keep in mind that you get only one shot with the Last Known Good Configuration feature. In other words, if it fails to revive your Windows XP on the first attempt, the backup copy is also corrupt.

#3: Use System Restore

Another tool that might be helpful when Windows XP won’t boot is System Restore. System Restore runs in the background as a service and continually monitors system-critical components for changes. When it detects an impending change, System Restore immediately makes backup copies, called restore points, of these critical components before the change occurs. In addition, System Restore is configured by default to create restore points every 24 hours.

To use System Restore, first restart the computer by pressing [Ctrl][Alt][Delete]. When you see the message Please select the operating system to start or hear the single beep, press [F8] to display the Windows Advanced Options menu. Now, select the Safe Mode item from the menu and press [Enter].

Once Windows XP boots into Safe mode, click the Start button, access the All Programs | Accessories | System Tools menu, and select System Restore. Because you’re running in Safe mode, the only option on the opening screen of the System Restore wizard is Restore My Computer To An Earlier Time, and it’s selected by default, so just click Next. Then, follow along with the wizard to select a restore point and begin the restoration procedure.

#4: Use Recovery Console

When a Windows XP boot problem is severe, you’ll need to use a more drastic approach. The Windows XP CD is bootable and will provide you with access to a tool called Recovery Console.

To boot from the Windows XP CD, insert it into the CD-ROM drive on the problem system and press [Ctrl][Alt][Delete] to reboot the computer. Once the system begins booting from the CD, simply follow the prompts that will allow the loading of the basic files needed to run Setup. When you see the Welcome To Setup screen, shown in Figure A, press R to start the Recovery Console.

Figure A

Recovery console

You’ll then see a Recovery Console menu, like the one shown in Figure B. It displays the folder containing the operating system’s files and prompts you to choose the operating system you want to log on to. Just press the menu number on the keyboard, and you’ll be prompted to enter the Administrator’s password. You’ll then find yourself at the main Recovery Console prompt.

Figure B

Choose your OS

#5: Fix a corrupt Boot.ini

As the Windows XP operating system begins to load, the Ntldr program refers to the Boot.ini file to determine where the operating system files reside and which options to enable as the operating system continues to load. So if there’s a problem rooted in the Boot.ini file, it can render Windows XP incapable of booting correctly.

If you suspect that Windows XP won’t boot because Boot.ini has been corrupted, you can use the special Recovery Console version of the Bootcfg tool to fix it. Of course, you must first boot the system with the Windows XP CD and access the Recovery Console as described in #4.

To use the Bootcfg tool, from the Recovery Console command prompt, type

Bootcfg /parameter

Where /parameter is one of these required parameters:

• /Add–Scans the disk for all Windows installations and allows you to add any new ones to the Boot.ini file.
• /Scan–Scans the disk for all Windows installations.
• /List–Lists each entry in the Boot.ini file.
• /Default–Sets the default operating system as the main boot entry.
• /Rebuild–Completely re-creates the Boot.ini file. The user must confirm each step.
• /Redirect–Allows the boot operation to be redirected to a specific port when using the Headless Administration feature. The Redirect parameter takes two parameters of its own, [Port Baudrate ] | [UseBiosSettings].
• /Disableredirect–Disables the redirection.

#6: Fix a corrupt partition boot sector

The partition boot sector is a small section of the hard disk partition that contains information about the operating system’s file system (NTFS or FAT32), as well as a very small machine language program that is crucial in assisting the operating system as it loads.

If you suspect that Windows XP won’t boot because the partition boot sector has been corrupted, you can use a special Recovery Console tool called Fixboot to fix it. Start by booting the system with the Windows XP CD and accessing the Recovery Console as described in #4.

To use the Fixboot tool, from the Recovery Console command prompt, type

Fixboot [drive]:

Where [drive] is the letter of the drive to which you want to write a new partition boot sector.

#7: Fix a corrupt master boot record

The master boot record occupies the first sector on the hard disk and is responsible for initiating the Windows boot procedure. The master boot record contains the partition table for the disk as well as a small program called the master boot code, which is responsible for locating the active, or bootable, partition, in the partition table. Once this occurs, the partition boot sector takes over and begins loading Windows. If the master boot record is corrupt, the partition boot sector can’t do its job and Windows won’t boot.

If you suspect Windows XP won’t boot because the master boot record has been corrupted, you can use the Recovery Console tool Fixmbr to fix it. First, boot the system with the Windows XP CD and access the Recovery Console as described in #4.

To use the Fixmbr tool, from the Recovery Console command prompt, type

Fixmbr [device_name]

Where [device_name] is the device pathname of the drive to which you want to write a new master boot record. For example, the device pathname format for a standard bootable drive C configuration would look like this:

DeviceHardDisk0

#8: Disable automatic restart

When Windows XP encounters a fatal error, the default setting for handling such an error is to automatically reboot the system. If the error occurs while Windows XP is booting, the operating system will become stuck in a reboot cycle–rebooting over and over instead of starting up normally. In that case, you’ll need to disable the option for automatically restarting on system failure.

When Windows XP begins to boot up and you see the message Please select the operating system to start or hear the single beep, press [F8] to display the Windows Advanced Options Menu. Then, select the Disable The Automatic Restart On System Failure item and press [Enter]. Now, Windows XP will hang up when it encounters the error and with any luck, it will display a stop message you can use to diagnose the problem.

#9: Restore from a backup

If you can’t seem to repair a Windows XP system that won’t boot and you have a recent backup, you can restore the system from the backup media. The method you use to restore the system will depend on what backup utility you used, so you’ll need to follow the utility’s instructions on how to perform a restore operation.

#10: Perform an in-place upgrade

If you can’t repair a Windows XP system that won’t boot and you don’t have a recent backup, you can perform an in-place upgrade. Doing so reinstalls the operating system into the same folder, just as if you were upgrading from one version of Windows to another. An in-place upgrade will usually solve most, if not all, Windows boot problems.

Performing a Windows XP in-place upgrade is pretty straightforward. To begin, insert the Windows XP CD into the drive, restart your system, and boot from the CD. Once the initial preparation is complete, you’ll see the Windows XP Setup screen (shown earlier in Figure A). Press [Enter] to launch the Windows XP Setup procedure. In a moment, you’ll see the License Agreement page and will need to press [F8] to acknowledge that you agree. Setup will then search the hard disk looking for a previous installation of Windows XP. When it finds the previous installation, you’ll see a second Windows XP Setup screen, as shown in Figure C.

Figure C

In-place upgrade

This screen will prompt you to press R to repair the selected installation or to press [Esc] to install a fresh copy of Windows XP. In this case, initiating a repair operation is synonymous with performing an in-place upgrade, so you’ll need to press R. When you do so, Setup will examine the disk drives in the system. It will then begin performing the in-place upgrade.

Keep in mind that after you perform an in-place upgrade or repair installation, you must reinstall all updates to Windows.

Part one

loadTOCNode(3, ‘moreinformation’); In part one, you start the Recovery Console, create a temporary folder, back up the existing registry files to a new location, delete the registry files at their existing location, and then copy the registry files from the repair folder to the System32\Config folder. When you have finished this procedure, a registry is created that you can use to start Windows XP. This registry was created and saved during the initial setup of Windows XP. Therefore any changes and settings that occurred after the Setup program was finished are lost.

To complete part one, follow these steps:

Note This procedure assumes that Windows XP is installed to the C:\Windows folder. Make sure to change C:\Windows to the appropriate windows_folder if it is a different location.

If you have access to another computer, to save time, you can copy the text in step five, and then create a text file called “Regcopy1.txt” (for example). To use this file, run the following command when you start in Recovery Console:

With the batch command in Recovery Console, you can process all the commands in a text file sequentially. When you use the batch command, you do not have to manually type as many commands.

Part two

loadTOCNode(3, ‘moreinformation’); To complete the procedure described in this section, you must be logged on as an administrator, or an administrative user (a user who has an account in the Administrators group). If you are using Windows XP Home Edition, you can log on as an administrative user. If you log on as an administrator, you must first start Windows XP Home Edition in Safe mode. To start the Windows XP Home Edition computer in Safe mode, follow these steps.

Note Print these instructions before you continue. You cannot view these instructions after you restart the computer in Safe Mode. If you use the NTFS file system, also print the instructions from Knowledge Base article KB309531. Step 7 contains a reference to the article.

In part two, you copy the registry files from their backed up location by using System Restore. This folder is not available in Recovery Console and is generally not visible during typical usage. Before you start this procedure, you must change several settings to make the folder visible:

These files are the backed up registry files from System Restore. Because you used the registry file that the Setup program created, this registry does not know that these restore points exist and are available. A new folder is created with a new GUID under System Volume Information and a restore point is created that includes a copy of the registry files that were copied during part one. Therefore, it is important not to use the most current folder, especially if the time stamp on the folder is the same as the current time.

The current system configuration is not aware of the previous restore points. You must have a previous copy of the registry from a previous restore point to make the previous restore points available again.

The registry files that were copied to the Tmp folder in the C:\Windows folder are moved to make sure that the files are available under Recovery Console. You must use these files to replace the registry files currently in the C:\Windows\System32\Config folder. By default, Recovery Console has limited folder access and cannot copy files from the System Volume folder.

Note The procedure described in this section assumes that you are running your computer with the FAT32 file system. For more information about how to access the System Volume Information Folder with the NTFS file system, click the following article number to view the article in the Microsoft Knowledge Base:

Part Three

loadTOCNode(3, ‘moreinformation’); In part three, you delete the existing registry files, and then copy the System Restore Registry files to the C:\Windows\System32\Config folder:

Note This procedure assumes that Windows XP is installed to the C:\Windows folder. Make sure to change C:\Windows to the appropriate windows_folder if it is a different location.

If you have access to another computer, to save time, you can copy the text in step two, and then create a text file called “Regcopy2.txt” (for example). To use this file, run the following command when you start in Recovery Console:

Part Four

loadTOCNode(3, ‘moreinformation’);

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