Managing Database Backup and Recovery - High-Performance Web Databases: Design, Development, and Deployment - page 697

to recover a database. In addition to having a backed-up system catalog, a
paper-based reproduction of the system catalog can be beneficial for audit
purposes or if the need ever arises to restore an older database backup on
a system prior to the installation of a new RAID array. As hardware is added
to the system, database load points will vary. This can have undesirable ef-
fects when loading an older version of a database back onto the server.

The user database catalog, on the other hand, is the definition of the
user database. It contains all the details regarding the tables and indexes
used in the physical implementation of the database and must be kept un-
der strict observance. It should follow a strict change control process and
must be backed up after each and every change to the database using a ver-
sion control system. A failure to backup the database catalogs will result in
loss of data if the database must ever be reloaded from flat files. The data-
base catalog, sometimes referred to as a schema, is the blueprint of the da-
tabase, the foundation of the database. It must be kept up to date, and its
path of evolution must be able to be retraced.

The data, of course, as the lifeblood of the database and the reason for
its existence, must also be safeguarded. The data should be backed up on
a daily basis, if time permits, but no less than once a week. Backups should
be restored from time to time to verify the validity of the backup and its
state. There is no point in performing a backup if it is not tested periodical-
ly. What may have been restorable last year may not be restorable now. Al-
so, recoverability from tape backups must be carefully tested.

Levels of Protection

Each of the individual methods provides a level of afforded protection.
The base level of protection and last line of defense for a system failure
should be a tape backup. This is deemed the last line of defense, as it is the
slowest of all methods to get the system back into operation when disaster
strikes. The highest level is a hybrid system. Exhibit 3 demonstrates the
varying levels of recovery and associated costs.

Exhibit 51-3. The varying levels of database recovery and associated costs.

Method Level Cost Downtime

Tape (mandatory) Low Low Hours

Mirroring Medium Low Minutes to hours

Duplexing Medium Low Minutes to hours

Data Guarding High Medium Minutes

Partitioning Medium High Minutes to hours

Replication High High Minutes

Clustering Very High Very High Seconds to minutes

Hybrid Combinations Extremely High Extremely High Seconds