Preparing for a Backup

When you create a backup, take the following precautions:

• Avoid using temp tables during heavy activity.
• Make sure you make sufficient logical-log space to create a backup.
• Keep a copy of your ONCONFIG file.
• Verify data consistency.
• Run the database server in the appropriate mode.
• Plan for operator availability.
• Synchronize with other administrative tasks.
• Do not use background mode.
• If necessary, label tapes appropriately.
• If necessary, prepare for writing to standard output.

The following sections address each of these topics.

Avoid Using Temp Tables During Heavy Activity

When you create a temp table during a backup while using the ontape utility, that table is placed in DBSPACETEMP. When heavy activity occurs during the backup process, the temp table can keep growing and can eventually fill up DBSPACETEMP. When this situation occurs, the backup aborts and your monitor displays a NO FREE DISK error message.

Making Sure Enough Logical-Log Space Exists

When the total available space in the logical log amounts to less than half a single logical-log file, the database server does not create a backup. You must back up the logical-log files and attempt the backup again.

You cannot add mirroring during a backup.

Keeping a Copy of Your ONCONFIG File

Keep a copy of the current ONCONFIG file when you create a level-0 backup. You need this information to restore database server data from the backup tape.

Verifying Consistency Before a Level-0 Backup

To ensure the integrity of your backups, periodically verify that all database server data and overhead information is consistent before you create a full-system level-0 backup. You need not check this information before every level-0 backup, but we recommend that you keep the necessary tapes from the most recent backup created immediately after the database server was verified as consistent. For information on consistency checking, see your IBM Informix: Administrator's Guide.

Using Online and Quiescent Backups

You can create a backup while the database server is online or in quiescent mode. The terminal you use to initiate the backup command is dedicated to the backup (displaying messages) until the backup completes. Once you start a backup, the database server must remain in the same mode until the backup finishes; changing the mode terminates the backup activity.

What Is an Online Backup?

You can use an online backup when you want your database server accessible while you create the backup.

Some minor inconveniences can occur during online backups. An online backup can slow checkpoint activity, and that can contribute to a loss in performance. However, this decline in performance is far less costly than the time that you lose when users were denied access to the database server during a backup.

During an online backup, allocation of some disk pages in storage spaces can temporarily freeze. Disk-page allocation is blocked for one chunk at a time until you back up the used pages in the chunk.

What Is a Quiescent Backup?

You create a quiescent backup while the database server is quiescent. Use quiescent backups when you want to eliminate partial transactions in a backup.

Do not use quiescent backups when users need continuous access to the databases.

Preparing to Backup to Tape

When you back up to tape, you must ensure that an operator is available and that you have sufficient media.

Ensuring That the Operator Is Available

Keep an operator available during a backup to mount tapes as prompted.

A backup could take several reels of tape. When an operator is not available to mount a new tape when one becomes full, the backup waits. During this wait, when the backup is an online backup, the physical log space could fill up, and that causes the database server to abort the backup. Thus, make sure an operator is available.

Labelling Tapes Created with ontape

When you label tapes created with ontape, the label must include the following information:

• Backup level
• Date and time
• Tape number that ontape provides

The following example shows what a label can look like:

Level 1: Wed Nov 27, 2001 20:45 Tape # 3 of 5

Each backup begins with its first tape reel numbered 1. You number each additional tape reel consecutively thereafter. You number a five-tape backup 1 through 5. (Of course, it is possible that you could not know that it is a five-tape backup until it is finished.)

Preparing to Back Up to Standard Output

If you choose to back up to standard output, you do not need to provide tapes or other storage media.

What is a Backup to Standard Output?

A backup to standard output creates an archive in the memory buffer provided by the operating system. Backing up to standard output has the following advantages:

• There are no expensive write and read operations to disk or tape.
• You can use operating system utilities to compress or otherwise process the data.
• You can use the archive to create a duplicate of the server by immediately restoring the data onto another database server.

If you back up to standard output, you must also restore from standard input.

Ensuring Adequate Memory

When ontape performs a backup to standard output, the data is written to an output file. The output file's directory must have enough disk space to hold the backed-up data. You can use operating system utilities to compress the data. In addition, the user executing the back-up command must have write permission to the file to which the backup is diverted or permission to create the file.