CompTIA Network+ All-in-One Exam Guide
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Figure 18-21 Incremental vs differential
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MON Full Backup
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TUE All Tuesday Changes
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Incremental WED All Wednesday Changes Differential WED All Changes Through Wednesday
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THU All Thursday Changes
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FRI All Friday Changes
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TUE All Changes Through Tuesday
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THU All Changes Through Thursday
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FRI All Changes Through Friday
Notice that a differential backup is a cumulative backup Because the archive bits are not set, it keeps backing up all changes since the last normal backup This means the backup files will get progressively larger throughout the week (assuming a standard weekly normal backup) The incremental backup, by contrast, only backs up files changed since the last backup Each incremental backup file will be relatively small and also totally different from the previous backup file Let s assume that the system is wiped out on a Thursday morning How can you restore the system to a useful state If you re using an incremental backup, you will first have to restore the last weekly backup you ran on Monday, then the Tuesday backup, and then the Wednesday backup before the system is restored to its Thursday morning state The longer the time between normal backups, the more incremental backups you must restore Using the same scenario, but assuming you re doing differential instead of incremental backups, you ll only need the weekly backup, and then the Wednesday backup to restore your system A differential backup will always require only two backups to restore a system (see Figure 18-22) Suddenly, the differential backup looks better than the incremental! On the other hand, one big benefit of incremental over differential is backup file size Differential backup files will be massive compared to incremental ones
Figure 18-22 Restoring from backups
Thursday: System Crash!
All Tuesday Changes
All Wednesday Changes
All Changes Through Wednesday
18: Network Management
One of the typical regimens or rotations for backing up to tape or to external hard drive and rotating media is called grandfather, father, son (GFS), and it works like this Typically, you d have a weekly and daily backup You run a full backup once a week and store the tape offsite Then you d run a differential backup each day The full backup is the father and the differential backups are the son The grandfather would be the last full backup of the month that then gets stored off site Using such a strategy enables you to restore by previous months, weeks, or days Choosing between incremental backups and differential backups is only one factor in choosing how you back up your data You must also consider your business, your data, your backup hardware, your operating systems, and other factors to create a backup strategy UPS An uninterruptible power supply (UPS) keeps your servers afloat in the event of an electrical brownout or blackout Without a good UPS, you simply cannot guarantee the proper level of uptime for your server A UPS enables a computer to function for a short period of time, but for extended outages, there s only one answer: backup generators NOTE Uninterruptible power supplies differ a lot in how they work and in the quality of electricity they provide The standard device we call a UPS is actually a standby power supply as it takes a few milliseconds to bring the power online These are fine and they are much cheaper than a true UPS Backup Generators For extended blackouts, such as when Hurricane Ike took down my office (and much of the Gulf Coast) for several weeks in 2008, you need to have backup generators to guarantee any hope of uninterrupted uptime A backup generator runs on some sort of fuel (often diesel or gasoline) to provide electricity RAID and Redundant Hardware Once you ve secured the electricity for your servers, you need to make sure that individual components within the system don t take out your entire server Most commonly, these redundant pieces of hardware include multiple hard drives, power supplies, and network connections As you most likely recall from studying for your CompTIA A+ exam, you can use two or more hard drives to provide fault tolerance through one of the several levels of Redundant Array of Independent Disks (RAID) There are three RAID levels commonly used in networks: RAID 0, RAID 1, and RAID 5 RAID 0 is known as striping Requiring at least two drives, RAID 0 breaks files into chunks called stripes and spreads the stripes across each drive in the RAID 0 array RAID 0 arrays are fast, but if one drive dies you lose everything There s no fault tolerance with RAID 0 RAID 1 is mirroring Again requiring at least two drives, RAID 1 makes a copy of every stripe and places it on each drive RAID 1 arrays have great fault tolerance, but because every file is copied twice they re slower than RAID 0 arrays