Setting the Tape-Device Parameters

You must consider the following points when you assign values to TAPEDEV and LTAPEDEV:

• Use separate devices, when possible.
• Use symbolic links.
• Specify remote devices.
• Specify /dev/null.
• Rewind tape devices.

The following sections explain each of these points.

Specifying Separate Devices for Storage-Space and Logical-Log Backups

When possible, the LTAPEDEV and TAPEDEV parameters in the ONCONFIG file must each specify a different device. When you specify separate devices for storage-space and logical-log backups, you can schedule these backups independently of each other. You can create a backup on one device at the same time you continuously back up the logical-log files on the other.

When the LTAPEDEV and TAPEDEV parameters specify the same device, the logical log can fill and cause the database server to stop processing during a backup. When this happens, you face limited options. You can either abort the backup to free the tape device and back up the logical-log files or leave normal processing suspended until the backup completes.

Precautions to Take When You Use One Tape Device

When only one tape device exists and you want to create backups while the database server is online, take the following precautions:

• Configure the database server with a large amount of logical-log space through a combination of many or large logical-log files. (See your IBM Informix: Administrator's Guide.)
• Store all explicitly created temporary tables in a dedicated dbspace and then drop the dbspace before backing up.
• Create the backup when low database activity occurs.
• Free as many logical-log files as possible before you begin the backup.

The logical log can fill up before the backup completes. The backup synchronizes with a checkpoint. A backup might wait for a checkpoint to synchronize activity, but the checkpoint cannot occur until all virtual processors exit critical sections. When database server processing suspends because of a full logical-log file, the virtual processors cannot exit their critical sections and a deadlock results.

Using Symbolic Links to Specify Tape Devices

You can specify the values of LTAPEDEV and TAPEDEV as symbolic links. Using symbolic links enables you to switch to other tape or tape-compatible devices without changing the pathname in the ONCONFIG file. For example, you can specify the following symbolic link for tape device /dev/rst0:

ln -s /dev/rst0 /dbfiles/logtape

When you set the LTAPEDEV configuration parameter, as the following example shows, you can switch to a different device without changing the LTAPEDEV parameter:

LTAPEDEV /dbfiles/logtape

You only need to change the symbolic link, as the following example shows:

ln -s /usr/backups /dbfiles/logtape

A user with one tape device could redirect a logical-log backup to a disk file while using the tape device for a backup.

Specifying a Remote Device

You can perform a storage-space or logical-log backup across your network to a remote device attached to another host computer. You should not do a continuous backup to a remote device. To specify a tape device on another host computer, use the following syntax:

host_machine_name:tape_device_pathname

The following example specifies a tape device on the host computer kyoto:

kyoto:/dev/rmt01

For information on the tape size for remote devices, see Tape Size for Remote Devices.

Using /dev/null for a Tape Device

It is recommended that you do not use /dev/null as the device when backing up. However, when you specify /dev/null as a backup tape device, you can avoid the overhead of a level-0 backup that is required after some operations, such as changing the logging status of a database. Obviously, you cannot restore storage spaces from a backup to /dev/null.

You can specify /dev/null as a tape device for logical-log backups when you decide that you do not need to recover transactions from the logical log. When you specify the tape device as /dev/null, block size and tape size are ignored. If you set LTAPEDEV either to or from /dev/null, you must use ON-Monitor or restart the database server for the new setting to take effect.

Setting TAPEDEV to stdio

To configure ontape to read from stdin or write to stdout, set the TAPEDEV configuration parameter to STDIO.

Rewinding Tape Devices Before Opening and on Closing

With ontape, you must use rewindable tape devices. Before reading from or writing to a tape, the database server performs a series of checks that require the rewind.