| The purpose of performing backups is to be able to restore individual files or complete filesystems. How often to perform backups, what data should be backed up, and how to rotate backup media are issues that can be confusing. Exactly how often you should back up which files depends on your system hardware configuration and particular needs. The criterion is, "If the system crashes, how much work are you willing to lose, and how much data can you afford to lose?" Remember that in any computing environment, the most valuable component is data, which may be in the form of databases or source code. Ideally, you would want to back up all files on all filesystems every few minutes so that you would never lose more than a few minutes' worth of work. However, this approach is not practical, and there are other ways in which you can achieve this near real-time redundancy without doing backups. TIP One option to achieve (near) real-time redundancy, without installing special hardware, is to periodically mirror the data from your server to a backup server using rsync, a file-synchronization utility included with SLES. rsync is discussed later in this chapter.
To examine the problem another way, you might ask this question: "How often should I back up the files?" The more users you have on the system, the more often you should back up the filesystems. A common schedule is to perform a full backup once or twice a week and partial daily backups. In a full backup, every single file on every filesystem is backed up, regardless of its creation or modification time. A partial, or incremental, backup backs up only the files that have been created or modified since the last full backup. Your backup strategy should be based on a rotation scheme that guarantees complete recovery in case of a disaster, within a reasonable amount of time. The following discussion is based on tape rotation because the most commonly used backup medium today is tape; however, the same principle applies to other storage media, such as rewritable DVDs. A rotation system distributes both old and new information across several tapes to reduce the risk of being lost due to media failure. The backup and storage media type and media rotation method you choose should take the following into consideration: Backup time window How much of a time window do you have when files on the servers are not being updated? Unless your backup software is capable of backing up open files, you need to schedule the job for the time period when no files are being accessed, or you have to make a conscious decision to skip opened files. Amount of data and backup media throughput Both the amount of data to back up and the speed at which that data can be written to the backup media will have an impact on your choice of backup methods. For example, if you have 20GB of data to be backed up daily and your backup device is capable of storing only 1GB per hour, performing a full backup will take at least 20 hours. In such a case, doing a daily full backup is probably not a good option, and a daily incremental backup is more appropriate. Media realibility and capacity Tapes have a limited lifetime, and constant use of the same tape can shorten its life span. The typical shelf life of a magnetic tape is just a few years, or shorter depending on the environment in which it is stored. On the other hand, tapes have a much higher storage capacity (hundreds of gigabytes) than most other storage media, such as CDs (800MB) and DVDs (48GB). Some installations use tapes for short-term storage, and when certain data needs to be retained for an extended period of time, this data is transferred to DVDs. In some cases due to audit requirements, data is backed up directly to CDs or DVDs so they cannot be subsequently modified. File restoration decision How many tapes (that is, how much time) will you need to restore information in the event of a complete system failure? (Always plan for the worst-case scenario, but hope for the best.) Also, the procedure to restore your filesystems back to a known state (for example, last night) should be as straightforward as possible; you should not have to run through 20 tapes for the task. Storage facilities It is general practice that backup archives should be sent to offsite storage facilities for safe-keeping. However, this may not be feasible in all cases, especially with smaller companies. In such instances, you need to consider the question "How many tapes are you physically capable of storing safely and securely onsite?" For instance, your backup tapes should be locked in a fire-resistant safe (one especially designed for tapes and magnetic storage media, and not one for documents). That way, should there be an accidental small fire, the safe can keep the tapes from melting for a few hours and at the same time secure them from theft. However, a typical office safe (22x18x18 inches) has only a small useable storage space (perhaps a couple of cubic feet) due to the thick walls. Alternatives include using bank safety deposit boxes, branch office locations, and even sending your backups home with the company's owner on a frequent basis.
You can choose a number of commonly used media rotation schemes for your backup needs. Two of them, the Grandfather-Father-Son (sometimes known as GFS) and the Tower of Hanoi methods, are discussed in the next section. Because these two rotation systems are based on the traditional incremental and differential backup methodologies, the concepts behind incremental and differential backups are discussed first. |
