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.
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.
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.
6.3.1.7. Decision 3: Decide on media rotation and storage
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 $100.
For tape and DVDRW media, you'll need to decide on your media rotation strategy. This is a compromise between the number of tapes/discs and how far back in time you wish to restore.
A simple rotation scheme involves buying a set amount of media and rotating through it. For example, 20 discs or tapes used only on weekdays will enable you to restore files to the state they were in during any weekday in the preceding four weeks.
A multilevel scheme permits you to go back farther in time. A simple three-level scheme (known as
Table 6-6. Grandfather/Father/Son backup scheme with 20 discs/tapes
| Level | Media used | Discs or tapes required |
|---|---|---|
| A (Son) | Monday Thursday | 4 |
| B (Father) | Three out of every four Fridays | 3 |
| C (Grandfather) | Fridays not covered by level B | 13 |
This scheme uses the same 20 discs or tapes, but permits you to restore to:
? Any weekday in the preceding week
? The end of any week in the preceding four weeks
? The end of any four-week period in the preceding year
Note that level A media will be more frequently used than level B or C media and will therefore need to be replaced more often.
You must also decide where and how you will store your media. Unless the media is stored offsite, a disaster such as fire or theft could result in the loss of both the original storage drives and the backup media, but storing media offsite will slow the restoration process.
6.3.1.8. Simple backup labeling
There are many ways of labeling backups, but one of the easiest is to create a file named
# touch /system-$(hostname)
# ls -l /system-*
-rw-r--r-- 1 root root 0 Jul 1 01:34 /system-bluesky.fedorabook.com
This will identify the originating system name as well as the date and time of the backup (from the file timestamp).
