The simplest scheme is to simply run a full backup of each system you're looking to protect every night. Full backups are easy to restore and easy to make, as long as the amount of data to back up fits on a single tape or on the set of tapes in a changer, and the backup can be completed in the available backup window.

Most of us started off using a basic backup scheme that made a full backup to a fresh tape nightly, recycling the Monday through Thursday tapes each week and holding the Friday tapes for a year or so. While this simple scheme appears to let you restore any file a user may delete or damage during the year, the sad truth is that a file created or downloaded on Monday and deleted on Thursday will be gone as soon as the Thursday night backup runs again. Administrators running versions of this scheme and looking for a more reasonable 30-day restore horizon use four sets of weekday tapes.

The volume of nightly backups could make running a full backup every night unreasonable because it exceeds the space available on your backup media or the time available in the backup window. When this happens, most administrators switch their daily backups to include incremental or differential backups. An incremental backup includes all the files that have changed since the last backup. A differential backup backs up just those files changed since the last full backup, so any changed file appears only on one differential backup.

When backing up online databases, such as SQL servers or messaging systems, incremental and differential backups include the transaction logs, which can be replayed to bring the database up to date. The backup system deletes or flushes posted logs when making an incremental backup and leaves them when making a differential backup.

Rebuilding a dead server from a set of conventional full and incremental backups can be tedious. You have to install the operating system on a new server, restore the full backup and then restore each of the incremental backups in turn, though some products, such as Legato Systems NetWorker, can make use of the full and incremental tapes at the same time.

Another little-considered problem with full and incremental backups is that in the event of a server rebuild, any file that existed on the server overnight since the last full backup gets restored, even if the user deleted it the next morning. We've even seen a situation where an administrator ran out of disk space while restoring the fifth of six incremental backups during recovery of a crashed server. The most sophisticated products, such as Legato NetWorker, keep track of deleted files to avoid just this problem.

Although they may take up more time and tape space, differential backups are significantly easier to use to rebuild a server. After reinstalling the OS, you need only the last full backup and the last differential to do a restore.

Some backup applications, including IBM's Tivoli Storage Manager and Dantz Development Corp. Retrospect, perform continuous or progressive backups, keeping a database of the files that exist on each system and their locations on the backup media. After an initial full backup, these systems perform incremental backups on a regular basis. They can then perform a full, or point in time, restore by building a list of files to be restored from the database and restoring the files from the backup media. Progressive backups minimize the server time and bandwidth required for a comprehensive backup regime and the media or storage space needed.

Tivoli Storage Manager extends this feature still further by letting you create a set of tapes containing the same files that would be on a conventional full backup made yesterday. These point-in-time copies are great for off-site backups. You can also tell Storage Manager to keep up to three past versions of every file and to keep old versions for seven days.

Retrospect's special take on progressive backups is to use the whole set of file descriptors in a volume's directory, including file size, modified date and creation date, to determine the files to be included in an incremental backup. This lets Retrospect store a single copy of files that exist on multiple client systems (single-instance storage) and make progressive backups -- or, as Dantz calls them, "incremental plus" -- to multiple backup sets. Given our general paranoia level and experience with failed backup media, we're never really secure until our data are recorded on two different sets of media, so we liked this feature.