Valores Y Estándares

The following fault tolerance measures are the typical measures used to mitigate the causes of loss listed in the first section of this chapter. Some of these measures are detailed here, while others that are covered in other chapters are merely mentioned here along with a reference to their respective chapters.

Backups

Backups are the most common specific form of fault tolerance and are sometimes naively considered to be a cure-all for all types of loss. Backups are simply a snapshot copy of the data on a machine at a specific time, usually when users are not using the system.

Traditionally, backups are performed to a tape device, but as disks have become less expensive, they have begun to replace tape as backup devices.

Traditional backup works like this: Every night, you insert a fresh tape into your server. The next morning when you arrive at work, you remove the tape, mark the date, and store it in your tape vault. At larger companies, you’ll never use that tape again—it’s a permanent record of your network on that date. In smaller companies, that’s the same tape you use every Wednesday, and you only keep tapes made over the weekend or perhaps you reuse them once a month.

Nearly all operating systems, including all Microsoft operating systems and all versions of Unix, support a backup methodology called archive marking, which is implemented through a single bit flag attached to every file as an attribute. The archive bit is set every time a file is written to and is only cleared by archive software. This allows the system to retain a memory of which files have changed since the last backup.

archive marking

A method used by operating systems to indicate when a file has been changed and should thus be included in an incremental backup.

Windows and Unix both come with simple tape backup solutions that are capable of

performing full and incremental system backups to tape or disk (except Windows NT prior to Windows 2000, which can only backup to tape) on a regularly scheduled basis. In Windows, the tool is called NTBACKUP.EXE, and in UNIX the tool is called "tar" (Tape Archive). (Tar is also commonly used to distribute software in the Unix environment.) Both applications work similarly; they create a single large backup file out of the set of backed up directories and files, and write it to tape or to a file on disk.

With effort, you can do anything you need with the built-in backup tools for these operating systems. But larger sites will prefer to automate backup procedures with enterprise backup software that can automatically backup multiple servers to a central archive server.

Tip You can script your own custom backup methodology using file copy programs like tar (Unix) and XCOPY (Windows) to backup files as well. Both programs can be configured to respect archive marking.

Most backup software offers a variety of backup options:

Full Backup Archives every file on the computer and clears all the archive bits so that all future writes will be marked for archiving.

Copy Backup Archives every file on the computer without modifying the archive bit flags. Copy operations proceed faster and can archive read-only files since the file does not have to be opened for write operations to reset the archive bit flag.

Incremental Backup Archives every file that has its archive bit set (meaning it has changed since the last backup) and resets the bit so that the next incremental backup will not re-archive the file.

Differential Backup Archives every file that has its archive bit set, but it does not reset the bit; therefore, every differential backup tape includes the complete set of files since the last full system backup.

Periodic Backup Archives all files that have been written to since a certain date.

Because software vendors have begun to realize how badly traditional solutions perform for restoration, a new type of tape backup called image backup has become available. In an image backup, a complete sector copy of the disk is written to tape, including all the information necessary to reconstruct the drive’s partitions. Because the backup occurs below the file level, image archives are capable of archiving open files.

Restoration is where an image backup shines. The image backup software will create a set of boot floppies for emergency restoration. By inserting the emergency restore boot floppy and an image tape, the computer will boot a proprietary restore program that simply copies the image on the tape back to disk. One reboot later and you’re looking at your old familiar computer.

Image backup is not for archiving—file access is not as good as traditional backup software, and in many cases is not available at all. But there’s no reason you can’t use different

software for archiving and backup. Tape Hardware

Tape devices range from simple single cartridge units to sophisticated robotic tape changers. Tape auto-changers are devices that use some mechanical method to change tapes among a library of installed cartridges. When one tape is filled to capacity, the next tape in the changer is installed, and the archive operation proceeds. With auto-changers, literally any amount of data can be archived. They suffer from the problem that the archive operation takes as long as there are cartridges to be used, because the operation is sequential, and the mechanical

devices used to change tapes are (as are all moving devices) subject to failure. Auto-changers frequently can take more time to perform an archive than is allotted because of the volume of information involved and their sequential nature.

Redundant Arrays of Independent Tapes (RAIT) is the latest development in archiving technology. This technology, also called TapeRAID, is an adaptation of disk RAID technology. RAIT uses as many tape devices in parallel as your backup problem requires. RAIT is usually cheaper and always faster than tape auto-changers because auto-changers are low-volume devices that are always expensive, and individual tape units are relatively

inexpensive. They are faster because the archival operation operates simultaneously. It takes only the time that a single tape archive takes, no matter how many devices are involved. Problems with Tape Backup

The problem with using tape for archiving and backup is that it is not reliable—in fact, it’s highly unreliable. You may find this shocking, but two-thirds of attempts to completely restore a system from tape fail. That’s an awfully high number, especially considering how many people rely upon tape as their sole medium of backup.

Humans are the major cause of backup failure. Humans have to change that tape every day. This means that in any organization that doesn’t have a dedicated tape archivist, the

overburdened IS team is bound to forget. And if you’ve tried to train a non-IS employee to change the tape, you probably feel lucky if it happens at all.

One of two things will occur when the backup software detects that the tape has not been changed. Poorly designed or configured software will refuse to run the backup in a misguided attempt to protect the data already on the tape. Better-configured software will simply

overwrite the tape assuming that a more recent backup is better than no backup at all. So in many cases, the same tape may sit in a server (wearing out) for days or weeks on end, while business goes by and everyone forgets about the backup software.

Note An individual tape cartridge is only reliable for between 10 and 100 uses—and unless you verify your backups, you won’t know when it has become unreliable. Be certain that your tape rotation policy only re-uses tapes ten times (or the manufacturer recommended amount) before they are discarded.

It is a combination of tape wear, truculent backup software, and this human failure component that contribute to the high failure rate of tape restorations.

A typical restore operation is very problematic. Assuming the worst—you lost your storage system completely—here’s what you have to look forward to: After installing new hard disks, you must reinstall Windows or Unix from scratch. Then you must reinstall your tape backup software. Once you’ve finished these tasks (after a frantic search for the BackupExec or ARCserve installation code that is required to reinstall the tape software and a panicked call to their tech support to beg forgiveness, mercy, and a new code number), you’re ready to

completely overwrite the installation effort with a full restoration from tape. You now get to sit in front of your server providing all the base system tapes, then the Monday incremental tape, the Tuesday incremental tape, and so forth until you hit the current day of the week—the whole time cursing your decision to use daily incremental backups. Once you’re completely finished, and assuming that all six tapes involved worked flawlessly, you’re ready to reboot your server—an entire work day after you began the restore operation.

Backup Best Practices

Backup is a critical security component of any network. Allocate a large enough budget to do it correctly.

Use tape devices and media large enough to perform an entire backup onto a single tape. In the event that this is not possible, use RAIT software to allow the simultaneous unattended backup of the entire system.

Always set your tape backup software to overwrite media that may have been left in the machine, without having to ask you to change or overwrite.

Choose image backup software rather than file-based backup software. Restorations are far easier and faster with this software.

Turn off disk-based catalogs. They take up far more space than they’re worth, and they’re never available when the computer has crashed. Use media-based catalogs that are stored on tape.

Tip Sony has a new advanced tape system that stores file catalogs in flash memory on the tape cartridge, providing instance catalog access with the media and eliminating the need for disk-based storage.

Perform a full-system backup every day. Differential, incremental, and daily backups that don’t create a complete image cause headaches and complications during a restoration operation and increase the likelihood of failure by adding more components to the process. If your backup system is too slow to backup your entire data set in the allotted time, get a new one that is capable of handling all your data in this time frame.

Use software with an open-file backup feature to backup opened files or force them closed if you perform your backup at night. Use the Windows “force system logoff” user policy to shut down user connections at night and force all files to close just prior to the backup.

If you reuse tapes, mark them each time they’ve been written to. Discard tapes after their 10th backup. Saving a few dollars on media isn’t worth the potential for loss.

If you haven’t implemented online archiving, pull out a full system backup once a week (or once a month at the longest) and store it permanently. You never know when a deleted file will be needed again.

Test your backups with full system restores to test servers at least once per quarter. This will help you identify practices that will make restoration difficult in an emergency.

Don’t bother backing up workstations. Rather, get users comfortable with the idea that no files stored locally on their computers will be backed up—if it’s important, put it on a network file server. This reduces the complexity of your backup problem considerably. Workstations should contain operating system and application files only, all of which can be restored from the original software CD-ROMs.

Use enterprise-based backup software that is capable of transmitting backup data over the network to a central backup server. Watch for network capacity, though, because that much data can often overwhelm a network. Schedule each server’s transmission so it doesn’t conflict when running over the same shared media as other servers do. You should put your archive server on your backbone or at the central point of your network.

You don’t have to spend a lot of money on esoteric archive servers, even for large environments. When you consider that a good 80GB DLT drive is going to cost $2,000, adding another $1,000 for a motherboard, hard disk, RAM, network adapter, and a copy of your operating system isn’t all that big of a deal. The software you have to install is likely to cost more than all the hardware combined anyway. So feel free to have six or eight computers dedicated to large backup problems. They can all run simultaneously to backup different portions of your network without investing in expensive RAIT software or auto-loading tape devices. You’ll save money and have a more standard solution that you can fix.