Save-while-active concepts

How it works

i5/OS objects consist of units of storage, which are called pages. The system maintains two copies of the pages of the object that change while you are performing the save operation:

• The first image contains the updates to the object with which normal system activity works.
• The second image is an image of the object at a single point in time. The save-while-active job uses this image to save the object to the media.

In other words, when an application makes changes to an object during a save-while-active job, the system uses one image of the object's pages to make the changes. At the same time, the system uses the other image to save the object to the media. The image that the system saves does not have the changes you made during the save-while-active job. The image on the media is as it existed when the system reached a checkpoint.

Checkpoints

The checkpoint for an object is the instant in time that the system creates an image of that object. The image that the system creates at that instant in time is the checkpoint image of the object.

Creating a checkpoint image is similar to taking a photograph of a moving automobile. The point in time that you took the photograph equates to the checkpoint. The photograph of the moving automobile equates to the checkpoint image. When the system has finished making the checkpoint image of the object, the object has reached a checkpoint.

Despite the name save-while-active, you cannot change objects while the system obtains their checkpoint images. The system allocates (or locks) objects as it obtains checkpoint images. After the system obtains the checkpoint images, you can change the objects.

Synchronization

When you save more than one object, you must choose when the objects will reach a checkpoint in relationship to each other. This is synchronization. The following are the three kinds of synchronization:

Full synchronization

With full synchronization, the checkpoints for all of the objects occur at the same time. The checkpoints occur during a time period in which no changes can occur to the objects. IBM® strongly recommends that you use full synchronization, even when you are saving objects in only one library. You also have the option of synchronizing data that is saved from multiple save operations.

Library synchronization

With library synchronization, the checkpoints for all of the objects in a library occur at the same time.

System-defined synchronization

With system-defined synchronization, the system decides when the checkpoints for the objects occur. The checkpoints for the objects might occur at different times resulting in complex restore procedures.

Save-outage time

The amount of time during the backup process that you cannot use the system is the save-outage time. You can use the save-while-active function to reduce or eliminate your save outage.

The easiest and recommended way to use the save-while-active function is to reduce your save-outage time. You can reduce your save-outage time by ending your applications that change objects. You can restart the applications after the system has reached a checkpoint for those objects. You can choose to have the save-while-active function send a notification when it completes the checkpoint processing. After the save-while-active function completes checkpoint processing, it is safe to start your applications again. When you use the save-while-active function in this way, the save-outage time can be much less than with normal save operations.

You can also use the save-while-active function to eliminate your save-outage time. When you use the save-while-active function to eliminate your save-outage time, you do not end the applications that make changes to the objects you save. However, the save operation affects the performance and response time of your applications. You should also use journaling or commitment control for all of the objects you are saving when using save-while-active in this way. Using the save-while-active function to eliminate your save-outage time might also greatly increase the complexity of your recovery procedures.

Save-while-active commands

The save-while-active function is an option on the i5/OS save commands listed below: