Bài giảng Network - Chapter 16

Chapter Overview BackupsAntivirus PoliciesPatches and Updates 1What Is a Backup? Backups are copies of data that you make on a regular basis, allowing you to restore lost data in a timely manner.Even if you have other storage technologies in place that provide fault tolerance, you still need a backup solution.Networks both complicate and simplify the process of making regular backups. More complicated because you have data stored on multiple devices that must be protected Simpler because you can use the network to access those devices A network backup strategy specifies what data you back up, how often you back it up, and what medium you use to store the backups.The backup hardware, software, and administrative policies you will use depend on how much data you have to back up, how much time you have to back it up, and how much protection you want to provide. 2Selecting a Backup Drive Try to automate as much of the backup process as possible.Select a device that can store all of your data without frequent media changes.Consider the speed at which the drive writes data to the medium.The amount of time that you have to perform your backups is sometimes called the backup window.The backup device that you choose should depend in part on The amount of data you have to protect The amount of time that you have to back it up 3Determining Backup Costs Cost is always a factor in selecting a hardware product. Faster drives are generally more expensive.A low-end backup drive can cost $100 to $200, which is suitable for backing up a home computer where speed is not a major factor.High-end backup drives can have prices that run into five figures. When you evaluate backup devices, be aware of the product’s media costs as well.Some products might seem economical because the drive is inexpensive, but in the long run they might not be, because the media are so expensive.Determine the cost per megabyte of the storage a drive provides.Divide the price of the medium by the number of megabytes it can store, and use this figure to compare the relative cost of various devices. 4Selecting a Drive Interface Backup devices can use any of the standard computer interfaces: Integrated Drive Electronics (IDE), universal serial bus (USB), or Small Computer System Interface (SCSI).The most common interface used in high-end network backup solutions is SCSI. SCSI devices operate more independently than IDE devices, which means that the backup process is more efficient.SCSI devices can maintain a queue of commands that they have received from the host adapter and can execute them sequentially and independently. When multiple IDE devices share a channel, only one operates at a time. Magnetic tape drives require a consistent stream of data to write to the tape with maximum effectiveness. 5A Typical Magnetic Tape Drive 6Magnetic Tape Drives Magnetic tape drives are the most common hardware devices used to back up data. Tape drives are not random access devices.You cannot mount a tape drive in a computer’s file system, assign it a drive letter, and copy files to it, as you can with a hard disk drive. A special software program is required to address the drive and send the data you select to it for storage.Magnetic tape drives are well suited for backups, and they offer several advantages:FastHold a lot of dataLow media cost, often less than one-half cent per megabyte Tape drives are useless for anything other than backups. 7Magnetic Tape Technologies Type Tape Width Cartridge Size Capacity (Uncompressed) Speed Quarter-inch cartridge (QIC) 0.25 in. 4 × 6 × 0.625 in. (data cartridge); 3.25 × 2.5 × 0.6 in. (minicartridge) Up to 20 GB 2 to 120 MB/min. Digital audio tape (DAT) 4 mm 2.875 × 2.0625 × 0.375 in. Up to 20 GB 3 to 144 MB/min. 8 mm 8 mm 3.7 × 2.44 × 0.59 in. Up to 60 GB Up to 180 MB/min. Digital linear tape (DLT) 0.50 in. 4.16 × 4.15 × 1 in. Up to 40 GB Up to 360 MB/min. Linear tape-open (LTO), Ultrium media 0.50 in. 4.0 × 4.16 × 0.87 in. Up to 100 GB Up to 1920 MB/min. 8CD-ROM Drives Writable CD-ROM drives, such as compact disc-recordables (CD-Rs) and compact disc rewritables (CD-RWs), can be used as backup devices.Advantages The low cost of the media makes CDs an economical solution.Many computers already have CD-ROM drives, which eliminates the need to buy a dedicated backup drive. Disadvantages By backup device standards, the capacity of a CD is low, about 650 MB.For network backups, CD-ROMs are usually inadequate.Network backup software products usually do not recognize CD-R and CD-RW drives. 9Cartridge Drives Removable cartridge drives, such as Iomega’s Zip and Jaz drives, can be used for backups.Zip cartridges hold only 100 MB or 250 MB, which makes them less practical than CDs for backups.Jaz drives are available in 1-GB and 2-GB versions, which is sufficient for a backup device. Cartridge drives mount into a computer’s file system. You can assign a drive letter to a cartridge drive and copy files to it just as you can assign a drive letter and copy files to a hard drive. Disadvantage: the cost of the media is extremely high, making cartridge drives impractical for network backups. 10A Typical Autochanger 11Autochangers An autochanger is a unit that contains one or more drives and a robotic mechanism that swaps the media in and out of the drives. Some autochangers are small devices with a single drive and an array that holds four or five tapes. Others are enormous devices with as many as four drives and an array of 100 tapes or more. When a backup job fills one tape (or other storage medium), the mechanism extracts it from the drive and inserts another, and the job continues.If you buy a large enough autochanger, you can create a long-term backup strategy that allows backups to run completely unattended for weeks at a time.Autochanger prices can be astonishingly high, reaching as much as six figures in some cases. 12Target Selection The most basic function of a backup software program is to let you select what you want to back up, sometimes called the target.A good backup program enables you to select targets by selecting Entire computersSpecific drives on those computersSpecific directories on the drivesSpecific files in specific directories 13The Backup Dialog Box in the Windows 2000 Backup Program 14Full, Incremental, and Differential Backups A full backup copies the entire contents of a computer’s drives to tape or another medium.When you perform a full backup every day, most of the files you are writing to the tape are exactly the same as they were the previous day.Between full backups, administrators perform special types of filtered jobs that back up only the recently modified files. An incremental job backs up only the files changed since the last backup job of any kind.A differential job backs up only the files that have changed since the last full backup. The backup software filters the files for backup jobs by using a special file attribute called the archive bit, which is on every file on the computer. 15Archive Bits The archive bit for a file is activated by any application that modifies that file.When the backup program scans the target drive during an incremental or differential job, it selects for backup only the files with active archive bits.During a full backup, the software backs up the entire contents of a computer’s drives, and also resets the archive bit on all of the files.As work on the computer proceeds after the backup job is completed, files are modified, and the archive bits for the modified files are activated.The next day, you can run an incremental or differential backup job, which filters out all files that do not have an active archive bit. Compared to a full backup, an incremental or differential backup job is usually much smaller, so it takes less time and less tape. 16Incremental Jobs Incremental jobs reset the archive bits on the files they back up.When you run an incremental job, you back up only the files that have changed since the last backup, whether it was a full backup or an incremental backup. If you restore an entire computer, you must first perform a restore from the last full backup tape, and then restore each incremental job performed since the last full backup.The order of the restore jobs is essential if you want the computer to have the latest version of every file. Advantage: uses the least amount of tapeDisadvantage: lengthens the restore process 17Differential Jobs Differential jobs do not reset the archive bit on the files they back up. Every differential job backs up all of the files that have changed since the last full backup. Advantage: differential backups simplify the restore process.Disadvantage: differential backups use more tape, because some of the same files are backed up each day. 18Scheduling All backup products let you create a backup job and execute it immediately.The key to automating a backup routine is being able to schedule jobs to execute unattended.Scheduled jobs can run when the office is closed and the network is idle.Not all of the backup programs supplied with operating systems or designed for stand-alone computers will support scheduling, but all network backup software products do.Backup programs use various methods to automatically execute backup jobs.You specify whether you want to execute the job once or repeatedly at a specified time each day, week, or month.After creating a logical sequence of backup jobs that execute by themselves at regular intervals, you only need to change the tape in the drive each day. 19The Schedule Job Dialog Box in the Windows 2000 Backup Program 20Logging and Cataloging Most backup software products can maintain a log of the backup process as it occurs. You can often specify a level of detail for the log. You should periodically check the log, which tells you When selected files are skipped for any reason When errors occur on either the backup drive or one of the computers involved in the backup process Backup software programs also catalog the files they back up, which facilitates the process of restoring files later. The catalog is a list of every file that the software has backed up during each job.To restore files from the backup medium, you browse through the catalog and select the files, directories, or drives that you want to restore. 21Media Rotation Using new tapes for every backup job and storing them all permanently can become extremely expensive. It is more common to reuse backup tapes.To reuse tapes properly, you must carefully plan your media rotation scheme. The Grandfather-Father-Son media rotation scheme refers to backup jobs that run monthly, weekly, and daily, and it requires you to maintainA set of tapes for daily jobs, which you reuse every weekA set of weekly tapes, which you reuse every monthA set of monthly tapes, which you reuse every year When the software program implements the rotation scheme, it Provides a basic schedule for the jobsTells you what name to write on each tape as you use itTells you which tape to put in the drive for each job 22Restoring You must perform periodic test restores from your backup tapes or other media to ensure that you can recover any lost data. Even if all your jobs are completed successfully and your log files show that all of your data has been backed up, there is no better test of a backup system than an actual restore. Most of the restore jobs you will perform will be restoring only one or a few files that a user has deleted. A backup program with a database that lets you search for a specific file makes your job much easier and enables you to restore any file in minutes. 23The Restore Dialog Box in the Windows 2000 Backup Program 24Network Backup Functions Choose a backup software product that is designed for network use.A network backup software product differs from an application designed for stand-alone systems in that the network product can back up other computers on the network.A fully functional network backup product can back up drives anywhere on the network, plus important operating system features on other computers, such as the Windows Registry and directory service databases.Network backup products often have optional add-on components that let you perform specialized backup tasks, such as backing up live databases or computers running other operating systems. 25What Is a Virus? A virus is a software routine that is deliberately designed to attach itself to another piece of software on a computer, perform some preprogrammed activity, and spread to other computers on the network. The worst types of viruses are engineered to irretrievably destroy all or part of the data stored on the computer by wiping out hard drives.Potentially damaging programs such as viruses, Trojan horses, and worms can find their way onto a network through file downloads, e-mails, or even removable disks. Like biological viruses, computer viruses are designed to replicate themselves by infecting other pieces of software.A virus on an infected removable disk can migrate to the computer’s hard drive and infect the code on the hard drive. 26How Viruses Can Spread Throughout a Network 27Virus Types Boot sector virusesExecutable file virusesPolymorphic virusesStealth virusesMacro virusesWormsTrojan horses 28Preventing Virus Infections All users should be wary of removable disks from outside sources and particularly of files attached to e-mail messages.You should use antivirus software products to protect individual computers.A virus scanner works by examining files and searching for specific code signatures that are peculiar to certain viruses.The scanner has a library of virus definitions that it uses to identify viruses.To keep your computers fully protected, you must update the virus signatures for your program on a regular basis. Antivirus programs designed for use on networks do not provide greater protection against viruses, but they simplify the process of implementing the protection. 29Major Updates Even a computer with a relatively simple configuration can have many different software components that are regularly updated.Years ago, operating systems manufacturers would release many different software patches, each addressing a specific issue.Now manufacturers release groups of updates in a single package. This practice was pioneered by Microsoft with its Service Pack releases for Microsoft Windows NT. Each Service Pack release for a product contains a collection of patches and updates, all of which are applied by one installation program.Because the various patches have all been tested together, the operating system environment is consistent. 30Patches Between the releases of Service Packs or other major software updates, manufacturers may also make individual patches available.A patch is usually a small fix that is designed to address a highly specific problem.In some cases, manufacturers recommend that you install a patch only under certain conditions, such as when you are using a particular combination of components or when you are experiencing a specific error.If your environment does not qualify, do not assume that you should install the patch anyway, just to keep your software current. Read all of the release documentation and carefully follow the manufacturer’s instructions. 31Software Upgrades and Updates Software manufacturers typically release periodic upgrades.An update is usually a relatively minor release that addresses specific issues or provides modest enhancements. An upgrade is a major release that provides new features and capabilities.In most cases, patches and updates are free, but you must buy an upgrade.The cumulative cost of the upgrade process can be enormous.If you do not need new features, it might not be worth upgrading. 32Chapter Summary Backups Magnetic tape is the most popular storage medium for backups.Backup software enables you to select the data you want to back up and sends it to the device you use for backups.Daily backup jobs can be full backups, incremental backups, or differential backups.A good backup software program allows you to schedule jobs to execute at any time.Network backup software enables you to back up data from computers anywhere on the network, and might also provide optional features such as live database backups. 33Chapter Summary (Cont.) Antivirus policies Viruses are dangerous programs that can damage the data on a computer and spread to the other computers on a network.To protect your network against viruses, you must run antivirus software on every computer. Patches and updates Obtaining, evaluating, and deploying software patches and updates is an important part of the network administrator’s job.Software upgrades are major undertakings that can be extremely expensive and time-consuming. 34