Chris Tyler - Fedora Linux

Any data that you want to preserve must be backed up; usually, this does not include the operating system or applications, because you can reinstall those.

Table 6-5 lists some common system roles and the directories that should be considered for backup.

Table 6-5. Directories used for critical data storage in various common system roles

Generally, backup frequency should be decided based on how often (and when) the data changes, and how many changes you are willing to lose.

For example, printer configuration data may be changed only a few times a year, and losing the latest change won't cost much in terms of the work required to re-create that change. Word processing documents may be changed daily, and you may want to ensure that you don't lose more than one day's work (or even a half-day's work); on the other hand, orders on a busy web site may be received every few seconds, and you may decide that you can't live with the loss of more than a few minutes worth of data.

How long can you live without your data? The answer probably depends on regulatory and operational issues.

Some types of informationsuch as information about cross-border shipmentsmust be reported to government agencies on a daily basis, for example, and delays are penalized by fines of thousands of dollars per day. This puts a tremendous amount of pressure on the data-recovery process. On the other hand, personal music and photo collections may not need to be restored until weeks or months after the data loss.

Some types of data loss or corruption may not be realized until weeks, months, or years after they have occurred, while others will be immediately obvious. In some caseswhen data changes quicklyit may be necessary to be able to restore data to the state it was in on a specific date, while in other cases it's sufficient to be able to restore data to the state that it was in at the end of a particular month.

Files may be selected for backup on an incremental basisonly files that have been changed since the last backup are selectedor a full backup may be performed.

Incremental backups often require significantly less storage space than full backups when dealing with large sets of individual files such as word processing documents because the number of documents that are changed each day is usually fairly small. On the other hand, a small SQL update query may cause all of the files in a database to be modified, nullifying the benefits of incremental backup in that context.

An incremental backup scheme usually involves making full backups periodically and then making incremental backups until the scheduled time of the next full backup. Restoring from an incremental backup therefore requires you to restore a full backup, then restore all of the incremental backups from that point forward. Thus, the time required for a restore operation may be much longer than for a system that uses only full backups. Also, if one of the backups is unusable due to media corruption or damage, you will not be able to reliably perform a full recovery.

Given the choice between full and incremental backups, I recommend using full backups whenever practical.

Cost, capacity, and speed usually drive the selection of backup media. There are many options available:

DVDR/RW

DVD is an attractive medium. Fedora includes software to produce compressed optical discs that are automatically decompressed by the kernel when they are read. The compression ratio will depend on the type of data being backed up; text files may compress by 7590 percent, while data that is already in a compressed format (such as OpenOffice.org documents) may not compress at all. You can reasonably expect 50 percent compression for a typical mix of user files, and 75 percent for databases containing text data; that means a single-sided DVDR, which costs only a few cents and which has a nominal capacity of 4.7 GB (usable capacity of slightly over 4.3 GB), will hold 8+ GB of regular user files or 16+ GB of database files. DVD is also a fast, random-access medium.

CD-R/RW

Similar to DVD, with a lower storage capacity and wider deployment. Because higher-capacity DVDs are similarly priced (actually, cheaper in some jurisdictionssuch as Canadadue to music levies on CDs), DVDs are preferred except when backing up a device such as a laptop that has only a CD-RW drive.

Tape

Tape is by far the most economical choice for high-volume data backup (>10 GB uncompressed), but it still doesn't come cheap. Tape drives can cost more than the disk drives being backed up, and each backup tape can cost 2550 percent of the price of the corresponding disk storage. Tapes are also fairly slow during search and restore operations due to their sequential nature.

Disk

Hard disks can be used for data backup. USB drives are particularly convenient for this purpose, but removable drive trays can also be used with ATA or SATA drives. Hard drives are fast, but expensive and fragile.

Remote storage

Copying an archive of data to a remote system periodically.

Remote mirror

Making an immediate copy of all data written to the local disk drive provides the ultimate backup, but this approach is complicated and does not by itself guard against data corruption or accidental file deletion. For one approach to remote mirroring, see " . . . mirroring to a remote drive as part of a disaster-recovery plan? " in the "What About . . . " section in Lab 6.2, "Managing RAID ."

I'm going to focus on DVD and tape storage options in this lab.

When using DVDs, you have the option of selecting DVDR media, which can only be written once. This provides an inexpensive, compact, and permanent archive through time; assuming one disc per day, a year's worth of discs will take only about 4L of space and cost less than $100.

For tape and DVDRW media, you'll need to decide on your media rotation strategy. This is a compromise between the number of tapes/discs and how far back in time you wish to restore.

A simple rotation scheme involves buying a set amount of media and rotating through it. For example, 20 discs or tapes used only on weekdays will enable you to restore files to the state they were in during any weekday in the preceding four weeks.