Red Hat ENTERPRISE LINUX 4 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual

Also see for ENTERPRISE LINUX 4 - DEVELOPER TOOLS GUIDE: Installation guide Installation guide Tuning guide Installation guide Manual

Contents

166 Chapter 8. Planning for DisasterIncremental backups are used in conjunction with a regularly-occurring full backup (for example, aweekly full backup, with daily incrementals).The primary advantage gained by using incremental backups is that the incremental backups run morequickly than full backups. The primary disadvantage to incremental backups is that restoring any givenfile may mean going through one or more incremental backups until the file is found. When restoringa complete file system, it is necessary to restore the last full backup and every subsequent incrementalbackup.In an attempt to alleviate the need to go through every incremental backup, a slightly different ap-proach was implemented. This is known as the differential backup.8.2.3.3. Differential BackupsDifferential backups are similar to incremental backups in that both backup only modified files. How-ever, differential backups are cumulative — in other words, with a differential backup, once a file hasbeen modified it continues to be included in all subsequent differential backups (until the next, fullbackup, of course).This means that each differential backup contains all the files modified since the last full backup, mak-ing it possible to perform a complete restoration with only the last full backup and the last differentialbackup.Like the backup strategy used with incremental backups, differential backups normally follow thesame approach: a single periodic full backup followed by more frequent differential backups.The effect of using differential backups in this way is that the differential backups tend to grow abit over time (assuming different files are modified over the time between full backups). This placesdifferential backups somewhere between incremental backups and full backups in terms of backupmedia utilization and backup speed, while often providing faster single-file and complete restorations(due to fewer backups to search/restore).Given these characteristics, differential backups are worth careful consideration.8.2.4. Backup MediaWe have been very careful to use the term "backup media" throughout the previous sections. Thereis a reason for that. Most experienced system administrators usually think about backups in terms ofreading and writing tapes, but today there are other options.At one time, tape devices were the only removable media devices that could reasonably be used forbackup purposes. However, this has changed. In the following sections we look at the most popularbackup media, and review their advantages as well as their disadvantages.8.2.4.1. TapeTape was the first widely-used removable data storage medium. It has the benefits of low media costand reasonably-good storage capacity. However, tape has some disadvantages — it is subject to wear,and data access on tape is sequential in nature.These factors mean that it is necessary to keep track of tape usage (retiring tapes once they havereached the end of their useful life), and that searching for a specific file on tape can be a lengthyproposition.On the other hand, tape is one of the most inexpensive mass storage media available, and it has a longhistory of reliability. This means that building a good-sized tape library need not consume a large partof your budget, and you can count on it being usable now and in the future.