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• The various kinds of data your site will deal with

• The expected volatility of each type of data

• The backup frequency you require to feel comfortable with potential losses

• The political boundaries over which the data will be spread

Use this information to design your site’s storage architecture with backups and potential growth in mind. Most sites are not ready to place complete trust in snazzy “black boxes” such as RAID systems or Network Appliance snapshots. Keeping project directories and users’ home directories on a dedicated file server can make it easier to manage your data and ensure its safety.

Prepare for the worst

After you have established a backup procedure, explore the worst case scenario: your site is completely destroyed. Determine how much data would be lost and how long it would take to get your system back to life (include the time it would take to acquire new hardware). Then determine if you can live with your answers.

10.2 BACKUP DEVICES AND MEDIA

Since many types of failure can damage several pieces of hardware at once, backups should be written to some sort of removable media. For example, backing up one hard disk to another (although better than no backup at all) provides little protection against a controller failure. Companies that back up your data over the Internet have entered the scene over the last few years, but most backups are still stored locally.

Many kinds of media store data by using magnetic particles. These media are subject to damage by electrical and magnetic fields. Here are some specific hazards to avoid:

• Audio speakers contain large electromagnets; it’s not a good idea to store tapes on or near them. Even small speakers designed for use with computers can be hazardous.

• Transformers and power supplies (including UPS boxes) generate electromagnetic fields. The “wall warts” used to power many peripherals contain transformers.

• Hard disks and tape drives have motors and magnetic heads, and their cases are often unshielded. Drives in metal cases are probably safe.

• Monitors use transformers and high voltages. Many monitors retain an electrical charge even after being turned off. Color monitors are the worst. Never store tapes on top of a monitor.

• Prolonged exposure to the Earth’s background radiation affects the data on magnetic media, limiting its life span. All tapes will become unreadable over a period of years. Most media will keep for three years, but if you plan to store data longer than that, you should either use optical media or rerecord the data.

The following sections describe some of the media that can be used for backups. The media are presented roughly in order of increasing capacity.

Many tape drives compress data before writing it to tape, allowing more data to be stored than the tape’s nominal capacity would suggest. Manufacturers like to quote tape capacities in terms of compressed data; they often optimistically assume a compression ratio of 2:1 or more. In the sections below, we ignore compression and cite the actual number of bytes that can physically be stored on each piece of media.

The assumed compression ratio of a drive also affects its throughput rating. If a drive can physically write 1 MB/s to tape but the manufacturer assumes 2:1 compression, the throughput magically rises to 2 MB/s. As with capacity figures, we have ignored throughput inflation below.

Although cost and media capacity are both important considerations, it’s important to consider throughput as well. Fast media are more pleasant to deal with, and they allow more flexibility in the scheduling of dumps.

Floppy disks

Floppies are the most inconvenient way to store backups. They are slow and do not hold much data (up to 2.8MB, currently). Although the individual disks are cheap, they hold so little data that they are in fact the most expensive backup medium overall. Floppy disks only last for a couple of years; never use them for long-term