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Backup strategies

Full	Backs up all data. Takes the longest to backup, but fastest to restore. Clears archive bit on all backed-up files.
Incremental	Only backs up files added or changed since last backup. Quick to back up, but restore requires multiple tapes (last full backup tape plus all incremental tapes since the last full backup. When restoring incremental, always take care to restore the correct order, oldest to newest.). Clears archive bit on all backed-up files.
Differential	Only backs up files since last FULL backup. Each backup takes a little longer, but restore requires only two tapes (most recent full backup and most recent differential backup). Does not clear archive bit.

NEVER, NEVER, NEVER MIX INCREMENTAL AND DIFFERENTIAL!

GFS (Grandfather, Father, Son) Backup Rotation

This is the most common of all backup rotation systems.

Requires a minimum of 21 tapes (assuming a 5-day rotation; 4 daily, 5 weekly, and 12 monthly.) for a full year of backups. This number excludes any spare tapes (for the inevitable failures), and the cleaning tape.
Create a monthly backup (Grandfather). This is to be stored permanently offsite, never to be reused.
Create a full weekly backup (Father).
Create a differential or incremental daily backup (Son).
Daily and weekly backups are reused each week and can be stored on- or offsite.

The number of tapes a site/client needs will depend on the number of days in that client's backup rotation. For instance, if the client wants a quarterly rotation (based on a four-week month, four daily tapes (son), four weekly tapes (father) and four monthly tapes (grandfather) for a total of 16 tapes for a full year of backups) then that's what that client requires. Under the original GFS rotation, only three tapes were used (Day 1 [son], Day 2 [father] and Day 3 [grandfather], then the cycle would repeat). This type of rotation is not very practical for long-term data recovery.

Backup Drives

Quarter Inch Cartridge – Introduced in 1972, primarily for the UNIX market, QICs are cheap—cheapest of all listed here—and excellent for stand-alone PCs, but not the best choice for networks. They are slow and capacity is limited. The name is derived from the width of the tape—one quarter inch. The drive comes in two sizes:
- 3.5-inch mini cartridge that holds up to 20 GB
- 5.25-inch data cartridge that stores up to 50 GB.
Digital Audio Tape (DAT) – Standard for companies that require high-speed, reliable and simple storage devices. This drive uses the same technology found in a VCR. Comes in two formats:
- Digital Data Storage (DDS): The current DDS standard, DDS-4, uses a 120-meter cartridge tape that stores up to 40 GB.
- Digital Linear Tape (DLT) – High-capacity tape storage that stores up to 80 GB on a single tape.
Please note: DDS and DLT are often confused as competing storage formats. While DLT has greater capacity than DDS, they are both DAT storage media.
Travan – An enhancement of QIC, newer network storage versions hold up to 10 GB of compressed data.
Mammoth – Introduced in 1996 by Exabyte, three drives have a 20-GB capacity with a transfer rate of 3 MBps. Mammoth-2 tapes, introduced in 1999, offer a new technology called SmartClean. When the drive is dirty – a common problem with tape drives – the drive will clean itself without user intervention.

How the capacities compare:

Storage Medium	Capacity
Diskette	1.44 MB
iomega Zip	100 MB or 250 MB
CD-RW	600-700 MB (MB for MB, most expensive here)
iomega Jaz	1 GB or 2 GB
DDS-1	2 GB
DDS-2	8 GB
Travan	10 GB
DDS-3	12 GB
QIC 3.5-inch mini cartridge	20 GB
Mammoth	20 GB
DDS-4	40 GB
QIC 5.25-inch data cartridge	50 GB
DLT	80 GB

For an excellent Intel article on GFS, click here.
Here’s a Cramsession article on why backing up data is so important and another article on the Tao of backup.